I don't think that Tim Johnson will die. But now it's likely that he won't seek reelection in 2008. So it looks like 2008 will be between Congresswoman Stephanie Herseth and Governor Mike Rounds.
But if he is incapacitated and unable to vote, the DemocRATS will have 48 members and 2 independents to organize the Senate. There 49 Republicans. The Republicans would be in a position to shut down the Senate. The DemocRATS would be hurting their chances of holding onto the SD seat in 2008 if Johnson were unable to represent the state, but the DemocRATS block a replacement. Each state is entitled to equal representation in the US Senate. A prolonged absence would be a serious hardship for South Dakota.