Georgia's 10th - vacant due to death of Republican Rep. Charlie Norwood
http://en.wikipedia.org/wiki/Georgia%27s_10th_congressional_district_special_election%2C_2007California's 37th - vacant due to the death of Democrat Juanita Millender-McDonald
http://en.wikipedia.org/wiki/California%27s_37th_congressional_district_special_election%2C_2007Massachusett's 5th - Marty Meehan is resigning in July to become the next Chancellor of the University of MA. The election will be timed so a replacement is ready upon his departure
http://en.wikipedia.org/wiki/Massachusetts%27s_5th_congressional_district_special_election%2C_2007And these are just the official ones we know about. It's a good bet Rick Renzi (Puke-AZ) will be forced to resign shortly, and who knows how many corrupt Congresscritters will be gone between now and the end of the year