If you look at party ID for voters, it's 37 - 37 - 26 Independent (The first time since the poll began that as many people identified as Republican as Democrat)
However, if you look at ideology, it's 35% Conservative, 45% Independent, and 21% Liberal.
This shows that America is more Conservative than liberal, but does not necessarily associate Republican with Conservative (As they shouldn't, lately)
Thus, a candidate like McCain who is seen as a moderate Republican gets support from Conservative republicans (not counting extreme religious right), but also manages to appeal to that vast portion of people who identify as Democrats, but call themselves Conservative or Independent.
Triangulation theory, call Billy, he came up with it.