This is where it gets interesting. If the electoral college ties at 269-269, then the House of Representatives gets to select the president on a one state/one vote basis. Basically, all of the representatives from a state get together and make a decision on whic candidate to support. Essentially a vote.
Since most states have more Republican representatives than democratic ones, Mitt would probably be the victor in such a scenario.
However, the Senate gets to choose the Vice President, and since more states have Democratic senators than Republican ones, the Vice President would probably stay Biden.
That would be a hollow victory for Democrats, as the VP only has as much power as the President gives him (with the sole exception of breaking ties in the Senate).