With the current projections for a close election between George W. Bush and John Kerry on November 2, there are a large number of reasonable scenarios that result in no electoral majority (i.e. a tie). In the event of no electoral majority, the U.S. Constitution declares in Amendment XII that

“if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President. But in choosing the President, the votes shall be taken by states, the representation from each state having one vote”.

The Vice President is chosen by the Senate
“if no person have a majority, then from the two highest numbers on the list, the Senate shall choose the Vice-President;”

Below are fourteen maps, each with a 269-269 tie in the electoral college. These are permutations on changes to 12 states (OR, NV, AZ, NM, MN, WI, IA, OH, PA, WV, NH) and ME Congressional District 2. I’m certain that additional permutations are possible (for example, no scenarios below are shown with Kerry winning Missouri).

As a note, Congress could easily eliminate no electoral majority scenarios in future elections (due to a tie) by changing the size of the House of Representatives to an even number (edited due to error pointed out by Dennis).