Wow, Sen. Johnson is only 60, that isn't good.
Anyhow, personally, I wish him a speedy recovery and my thoughts and prayers go out to him and his family.
Politically, I can't help but wonder what impact this will have. Unless the stroke is so bad that he is physically unable to do his job, I'm imagining he'll hang on till the end of his term, but it'll make it much more likely that he'll retire in 2008. If that's the case, that might actually be bad for Republicans, because I think Johnson is easier to beat then Herseth would be.
Rounds could beat either Johnson or Herseth. If Rounds is smart, he would appoint an elder stateman Republican (why not John Thune's mentor, former U.S. Senator Jim Abdnor) to finish out the term, and then run for the seat himself in two years, turning the governorship over to his LG.
The LG would be running as an incumbent for governor, Rounds would take the high ground *and* be U.S. Senator for a very long time.
P.S. I, too, hope that this is all overblown and that he has a full recovery. Johnson may have had the election stolen for him against Thune, but it was Daschle doing the stealing. He's a far more decent sort than Daschle ever was, even though he was a wholly owned subsidiary of Daschle, Inc. But then, you had to be to get elected in SD as a Dem in those days.