The 12th amendment dictates that in the event of a tie in the electoral vote--269-269--the House of Representatives "shall choose immediately, by ballot, the President."
In deciding who to elect in the event of an electoral vote tie, Congressmen will be forced to consider who won the popular vote; in fact, the popular vote count might be the only politically palatable factor for Congress to consider. So remind your friends in non-purple states to vote even if that state's electoral college outcome seems foregone because there's a small but real chance that the popular count will determine the election (and if you're in California, don't forget to vote yes on proposition 1A because an SF-LA supertrain would be awesome).
Here's an article about some wild scenarios that could ensue in the event of a tie. For instance, Obama as President, and Palin as VP.