It is highly unlikely the Electoral College vote will end up in a 269-269 tie. And, although it's obviously more likely in a close race, if Barack Obama holds or widens his current leads, nationally and in the states, the possibility of a tie approaches zero. (Nate Silver's simulations at fivethirtyeight.com show a 3 in a thousand chance.)
But if McCain can tighten the race in the final four weeks, there are a variety of scenarios that could result in an exact tie. (One scenario is that Obama holds the 252 electors in the states John Kerry won, and adds the combined 17 electors from Iowa, Nevada and New Mexico. But there are others, especially when you consider that Maine and Nebraska elect all but two of their electors based on congressional district performance.)
In the case of a tie, the election is thrown into the newly elected House, with each state delegation casting one vote. That is, California's 53-member delegation gets one vote and so does Wyoming's at-large member.
Currently, that would probably still mean an Obama presidency because Democrats have majorities in 27 delegations, Republicans 21, with two split evenly. But, again, it is not the delegations of the current, 110th House that vote; it is the newly elected 111th House.
In his weekly National Journal column, Charlie Cook of the Cook Political Report walks us through what, based on current projections, we could expect. Short answer: still an Obama presidency. In other words, Obama's electoral threshold is 269, whereas John McCain will need that extra 270th elector.