Sure, it's 2 years before the 2008 Presidential election, so how could that happen?
Republicans drag their feet on FISA and National Security Act violations by Bush and Cheney, even while revelations pile up in the press about them. Nothing happens by November 2006, and popular outrage throws the Republicans out of office in the House, returning a Democratic majority, including a Democratic Majority Leader and (bingo!) a Democratic Speaker of the House.
Following swearing in (January 2007), the House draws up bills of impeachment on both Cheney and Bush which meet with the required votes; the Senate follows suit.
And by the end of February 2007, the Democratic Speaker of the House is President.
All it may take is a bit of foot-dragging by Congressional Republicans.