Republican Deborah Pryce is retiring in Ohio's fifteenth district, leaving the seat open for the taking by Mary Jo Kilroy. Pryce is leaving because she is an ally of Hastert, who is also retiring, and she doesn't want to go through a grueling reelection to sit in a minority irrelevant position in the House. The district split between Bush and Kerry, but I would expect it to move aggressively to the Democratic side this cycle after Democrats romped in Ohio in 2006.
Generically, no politician likes being in the minority, so a lot of Republicans will choose to retire over the next few years. That's what happened after the 1994 election, when a bunch of Democrats either retired or outright switched parties. Hastert went, now Pryce, and Virginia Senator John Warner's another one who looks likely to retire. There are more where that came from.