FiveThirtyEight.com breaks it down - Election 2008 will come down to VA, CO, PA, OH and NV

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This is beginning to look like a five-state election. Those states are Virginia, Colorado, Pennsylvania, Ohio and Nevada. Essentially all relevant electoral scenarios involve some combination of these five states.

...

If the national polls tighten significantly ... McCain will edge out a victory in North Carolina, Florida, Indiana, North Dakota, Montana, Georgia, and Missouri; put those states in the McCain column for the time being. Likewise, New Mexico, New Hampshire, Minnesota, Michigan, Wisconsin and Iowa all appear safe for Obama, even in the case of significant tightening. Put those in the Obama column.

That leaves our five states in play. The victory conditions for Obama involving these five states proceed something as follows:

1. Win Pennsylvania and ANY ONE of Colorado, Virginia, Ohio, or Nevada*

2. Win Ohio and EITHER Colorado OR Virginia.

3. Win Colorado AND Virginia AND Nevada.

(* Nevada produces a 269-269 tie, which would probably be resolved for Obama in the House of Represenatives.)

Now, suppose you think that Colorado is already in the bag for Obama because of his large edge in early voting there. We can then simplify the victory conditions as follows:

1. Win Pennsylvania

2. Win Ohio

3. Win Virginia AND Nevada

That's basically what it comes down to, although I'm sure each campaign would claim that there are a larger number of states in play.

http://www.fivethirtyeight.com/2008/11/todays-polls-111.html