Dem-leaning blogger Nate Silver, whose blog www.fivethirtyeight.com focuses heavily on statistical analysis, says "all relevant electoral scenarios involve some combination" of Virginia, Colorado, Pennsylvania, Ohio and Nevada.
He says "by far the most likely scenario is that Obama wins some relatively decisive victory of anywhere from 3-12 points in the popular vote" but adds that if the polls tighten and McCain makes it a close race, those five states assume more importance.
Here are the paths to victory he then sees for Obama:
1. Win Pennsylvania and ANY ONE of Colorado, Virginia, Ohio, or Nevada*Read his entire post
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 Representatives.)