-In some battleground states a third of all votes will be made early. Combined with military votes and other absentee voting I could see the exit polling showing McCain up 15% OR Obama up 15% -- it's just going to be too inaccurate to make any real sense of.
-In 2004, for whatever reason, Kerry was hugely over represented in exit polling. A lot of Bush voters either didn't stop and answer and/or they lied because they were ashamed of voting for him. The racism angle might play out the same this year. I've noticed that racists are often really ashamed of their views and try very hard to over compensate when they feel like they are being scrutinized.
So anyway, no one should get too excited or too depressed over exit polls this year. I suppose, given the choice, I'd rather see Obama lagging in exit polling to motivate the get out of the vote effort. The first leaks tend to happen mid-day so you've got 4-6 hours of east coast voting and 6-9 hours of west coast voting. A big Obama surge in the exit polls would probably be really bad. I don't think it will come down to winning NV but I'd definitely like to seal the deal there too just in case.