These aren't necessarily the states that will be entirely crucial to the whole election (as Florida was in 2000 and Ohio in 2004) but they are states which aren't considered "locks" and have potential to go either way as of this stage.
Here's a more in-depth look from the Washington Post: http://voices.washingtonpost.com/thefix/2008/08/friday_line_battleground_state.html
That article ranks each state by its potential to swing in this election from the way they voted in 2004, with Iowa (which voted Republican in 2004) considered the most likely to vote for Obama this time around, and Florida ranked 10th as the least likely to switch.
This recent New York Times article has a ranking of each state based on whether they're solidly on one side, leaning towards one side, or complete tossups:
http://elections.nytimes.com/2008/president/whos-ahead/key-states/map.html
That ranking has Nevada, New Mexico, Colorado and Ohio as the only true tossups. Also notice that the 270 electoral votes needed to win lie right within that "tossup" group, meaning that assuming all of the "leaners" stay on their current side, those 4 states will determine the winner.
Overall, it's hard to say at this point which states will be the battleground states which will determine the winner, since a lot can happen in the time between now and the election. However, it's safe to bet that a combination of the answers above will the states to watch on election day.