If you're going to keep track of possible electoral vote scenarios this fall, you'll want to bookmark this site which does a great job of following polls and showing you how everyone's doing. The caution is that they take even a one-point lead (in a poll with a margin of error greater than that) as significant.
For instance, at this moment (it could change within the hour), they have Obama at 317 and McCain at 221. Since you need 270 to win, that sounds like a landslide for Obama…but take a closer look. They have Virginia as "barely Democratic" and they award Obama its 13 electoral votes. Why? Because the Rasmussen Poll has Obama at 45% and McCain at 44%. The margin of error in this poll is 4.5% so a one point lead is meaningless. (Then again, Rasmussen is the poll most Republicans love. When all others have Democrats ahead, Rasmussen often favors the G.O.P., at least when it's far enough from election day that they won't be proven instantly wrong.)
Or take Nevada. The site gives its 5 electoral votes to McCain since McCain is at 44% and Obama's at 42% in a Mason-Dixon poll with a 3 point margin o' error. Obama gets Missouri's 11 and Ohio's 20 though he's one point ahead there while McCain gets North Carolina's 15 due to being two points ahead. And all of this is going by only one poll per state…the most recent one, even if one the day before said something very different.
Bottom line: Use it but click through on the swing states and look at all the polls for a given state and at the trends. In much of the country, it is already over: McCain ain't gonna win California or New York. Obama ain't gonna win Texas or Utah. And an awful lot of states could go either way.