It might be in bad taste... but the Washington Post is talking about it so....

I may as well bring it up:

Sen. Tim Johnson (D-S.D.) suffered a "possible stroke" today, and the prognosis for his recovery remains unknown, according to his office....

Should Johnson be unable to serve when the 110th Congress convenes in January, it could mean a 50-50 split in the Senate. Gov. Mike Rounds (R) would be tasked with appointing a successor to Johnson -- presumably a Republican. That could effectively put the Senate, which is slated to switch to Democratic control in January, in Republican hands because Vice President Cheney would cast the tie-breaking vote.

However, in modern history the Senate has never declared a seat vacant as a result of a senator's physical condition.

That last sentence is key.