Bruce Bartlett makes a good point, that having the House of Representatives in the hands of the party not in control of the Presidency can be good for the party that does control the Presidency. However, there are plenty of counter examples, where the party that won the Presidency was in charge of both the Presidency and the House. That includes Bush in 2004, Lyndon Johnson in 1964, and FDR in 1936, 1940, and 1944.
Let me further point out that there are examples of a party losing the Presidency despite the opposition party being in charge of the House. That would include Nixon's loss in 1960, Ford's loss in 1976 and Gore's loss in 2000.
Thus, the historical evidence does not lean heavily enough to one side to conclude that the probability of the GOP keeping the Presidency goes way up if the Democrats are in charge of the House.