After an absence of over a year -- and what a year it's been! -- I've decided to begin writing again and, hopefully, contribute something to the discussion.
In the wake of Barack Obama's victory in 2008, "realignment" became the conventional narrative among talking heads, often combined with its sister narrative, the death of conservatism/the GOP/the Reagan Revolution (take your pick). The problem with conventional wisdom is it usually treats a snapshot in time as a summation of all that is, will be, and ever was. And that very problem creates false baselines by which conventional wisdom measures future events.
Coming off two narrow victories by George W. Bush in 2000 (271 electoral votes to 266) and 2004 (286-251), the common understanding of the electoral map was that the country was divided in two: red states and blue states, and that only a small handful of states fell into the "swing" category and determined the outcome. In other words, the map looked something like this:

Flip the "swing state" of Ohio and John Kerry would have been president.
Using those two elections as the standard, Obama's 365 electoral votes in 2008 look like a major diversion from the norm.
However, the truth is that Bush's two elections were the anomaly. Take a look at the last pre-Bush election (1996):

Not only have the "swing states" of OH and FL flipped, but an entire axis stretching through the midwest from Canada to the Gulf of Mexico (with the exception of MN) is in Clinton's column, as are the three western states of NV, AZ and NM and Appalachian states TN, KY and WV.
(Edit: to clarify, obviously MN is in Clinton's column too. I meant that it was excepted from the other states in that it's never been considered a "red" state)
The electoral margin of victory in the five "pre-Bush" elections also suggests something very different than a "divided America":
- 1996 (Clinton-Dole): 379-159
- 1992 (Clinton-Bush): 370-168
- 1988 (Bush-Dukakis): 426-111
- 1984 (Reagan-Mondale): 525-13
- 1980 (Reagan-Carter): 489-49
In fact, to find a president elected with fewer than 300 electoral votes, you have to go back to Jimmy Carter's 1976 win over Gerald Ford (297-240). The next one that's even close is Richard Nixon's 1968 victory over Hubert Humphrey (301-191), and if one assumes that, but for the presence of George Wallace on the ballot, Nixon would have carried AR, LA, MS, AL, and GA, his victory jumps to 346-191.
To varying degrees, most elections have represented consensus on the part of the electorate (despite the protestations of the losing party and its followers), and 2008 simply returns us to that pattern. The 2000 and 2004 elections, rather than reflecting the "divided America" that was such a popular narrative (see the "snapshot" comment above), reflect more on the failure of Bush and his campaign to achieve consensus, rather than a unique level of division.
The point of all this is that in the long view, the reality is that "flexible independents" rule. By this I mean not just those who are not registered as Republicans or Democrats, but those who are generally inclined toward one party (and thus registered), but won't rule out voting for the other (think Reagan Democrats). These voters are largely working and middle class, and are particularly concerned with everyday-living issues such as jobs, education, and (when they feel it affects them directly, such as after 9/11) national security. One of the reasons Ohio goes with the winner so consistently is it represents a microcosm of these voters. They're not particularly ideological, and neither are the election results they produce, which is why both a very conservative (by 1980 standards) Reagan and a very liberal Obama can rack up such big wins. Once a "comfort threshold" is reached, the voters will hand them the keys to the White House, based on the belief that they're better equipped to manage those issues than the other guy.
It is within this prism that the question of "realignment" must be answered. I don't know if anyone's ever established a hard and fast definition of realignment, but in my mind it represents a long-lasting shift in a segment of the electorate -- geographic, demographic or otherwise -- from general fealty to one party to the other. Of course, the only way to measure if it's long-lasting is to see whether the shift holds through a string of elections. The most obvious example would be the shift of a stripe of southern states (MS, AL, GA, SC, and NC) from a Democratic lock through 1960 to reliable Republican territory (with the exception of GA, which went for favorite son Jimmy Carter and also Bill Clinton in 1992) for the 48 years that followed, losing only NC in 2008.
Here's the 2008 map:

(Note Nebraska, which apportions electoral votes, also awarded Obama 1 vote, not reflected visually)
With the longer pattern in mind, the only earth-shattering wins for Obama are IN, VA, NC, and possibly CO. I will leave to others to break down exit polls and try to read the tea leaves on whether Obama's wins in these states are indicative of a lasting lock by Democrats, but it seems unlikely. Increased turnout among African-American voters -- who already vote for Democrats in excess of 90% -- was crucial to Obama's win in NC. It's difficult to imagine the Democrats sustaining that turnout for just any Democratic candidate.
Much has also been made of Obama's success among younger voters, and indeed it was this success that allowed Obama to change the playing field and snatch the Democratic nomination from the "inevitable" Hillary Clinton, and at least in part helps account for the fact that in 2008 Obama netted the highest number of voters in American history. But it also remains to be seen if these voters will continue to turnout for Democrats generally in future elections, or if their loyalties will remain constant throughout their lives. It seems like a silly assumption that no 18-year-old Reagan voter in 1980 became a 30-year-old Clinton voter in 1992, or a 46-year-old Obama voter in 2008. Similar assumptions about Obama's youngest voters seem equally silly.
So was 2008 a realignment? In regard to a few states, the jury's still out, and will be until 2012 and beyond. However, given the 2008 election's similarity to most other presidential elections, it seems unlikely. Candidate Obama clearly sealed the deal for a consensus among the above-described "flexible independents", but the Democrats' string of losses in statewide elections in VA, NJ and MA since then suggest that his 2008 win isn't translating into large numbers of newly loyal Democrats.
It seems much more plausible that Obama ran a smart, effective campaign in a year when these "flexible independents" were already inclined to pass the baton to the Democrats, perceiving the Republicans as no longer competent to govern. Such power shifts, particularly among these voters, are almost always based on perceived competence, not ideology, which probably explains why so many of these same voters have bristled at Obama's attempts to pursue a Great Society-esque program of social welfare and government intervention in the economy.
It seems to me that how Obama and the Democrats adjust their governance to the concerns of the "flexible independents" will have far more bearing on their future electoral fortunes than any perceived shifts in the electorate in 2008.