Who is Dennis Daugaard? I think you're correct that Hersheth would be a formidable candidate in a special election, she could probably beat Rounds if he ran himself or any other Republican. Hopefully, Rounds, who is reportedly good friends with Johnson?, would put partisanship aside and appoint a Democrat to the seat. The people of South Dakota chose a Democrat at the last election and it would be subverting their will to replace a Democrat with a Republican. The people of the United States voted in November to give the control of the United States Senate to the Democratic party, and their votes should be respected.
Hopefully though none of this will be neccesary as Tim Johnson shall recover and go on to serve the people of South Dakota in the Senate for many years.
I hope that Johnson will continue serving in the Senate, and be re-elected in 2008. But the fact of the matter is, if he resigns, a Republican will be appointed. Just about any Governor would do the same for their party, especially if the majority was at stake. When Sen. Paul Coverdell (R-GA) died in 2000, Gov. Roy Barnes (D-GA) appointed Zell Miller to his seat. Regardless of how conservative Miller was, it was still a another seat for the Democrats.
It's happened so many times, it's just to be expected.