Senator Tim Johnson (D) of South Dakota suffered a stroke the other day, and they did brain surgery to correct a hemmorhage. It's not known to what extent he'll recover if he recovers at all or whether or not he'll still be able to serve in the senate.
As I understand it, if he's incapable of fulfilling his post, the governor of the state appoints a replacement. The governor is a Republican and would likely appoint a Republican. That would put the Senate split 50/50 between the Dems and the Republicans. In the event of a tie, Dick Cheney breaks it. So essentially, if home dude doesn't pull out, the hold the Democrats have on the Senate is done.
No idea what will happen. It's all pure speculation at this point. Strom Thurmond was basically a vegetable for the last years of his Senatorial service. We'll see.
http://www.cnn.com/2006/POLITICS/12/...ill/index.html