I've noticed that, unlike state-level polls, most of the national polls report "registered voters" rather than "likely voters." That's true of both today's ABC/Washington Post poll, for example, and last week's NBC/Wall Street Journal poll. Intuitively, this seems like it would understate the support of the candidate with the enthusiasm on his side--Obama in this case--since it wouldn't reflect the fact that more of their supporters are likely to turn out.
The Post, in its write up, seems to hint at this. Though Obama only leads McCain by a 49-45 margin among registered voters, the Post notes that:
But [McCain] starts that campaign with several deficits, including an enthusiasm gap. A majority of voters, 55 percent, said they are enthusiastic about Obama's candidacy, while 42 percent said the same for McCain. Three times as many said they are "very enthusiastic" about Obama as said so about McCain.
Even among McCain and Obama supporters, there is a clear difference in interest.
Ninety-one percent of Obama's supporters are enthusiastic about his candidacy, including 54 percent who are very enthusiastic. Fewer of McCain's backers are as ardent: 73 percent are enthusiastic about his run, but just 17 percent are very much so. There appears to be some leftover animosity toward him on the right. Overall, 13 percent of conservatives are very enthusiastic about McCain, compared with nearly half of liberals who feel as strongly about Obama.
In fact, I'm tempted to go further with this idea. A lot of people have interpreted the Post and Journal polls as evidence that Obama hasn't gotten much of a bounce since wrapping up the nomination. There's obviously something to this--he clearly isn't getting a massive bounce. But using registered voters instead of likely voters may also understate the change in Obama's support (i.e., the bounce), not just his overall level of support. That is, you would expect Obama's supporters--and Democrats in general--to become more enthusiastic about him after he became the nominee, even if they were pretty enthusiastic about him beforehand.
It's not exactly dispositive, but there's some support for this in the polling. Obama has increased his lead over McCain in the Journal poll by 3 points since late April, the last time it came out. Over the same period, he increased his lead by somewhere between 5 and 10 points in Rasmussen (depending on which edition of the poll you use--they come out pretty frequently). Rasmussen is one of the very few national polling outfits that reports likely voters. Similarly, Obama's lead over McCain in the WaPo is basically unchanged (down 1) since early April, the last time they reported a comparable number, but up 9-10 points in Rasmussen.
Obviously, you'd need to look at more polls before drawing a conclusion on this question. (I'll leave it to someone else to look back over previous presidential cycles.) But it wouldn't shock me if the polls that only report registered voters systematically understate bounces. It seems like that would almost be true by definition.