New Jersey was loose with the law and allowed Torricelli to be switched after the ballot deadline. Texas was tight with the law and refused to allow DeLay to be replaced. I can't say what Florida would do, but they sure were loose with the laws in 2000.
Also, the US Supreme Court refused to get involved in New Jersey. They may have second thoughts now that this is becoming a standard practice in more than one state -- to drive out an opponent after filing deadlines in order to win an unopposed race.
-PJ
There is a difference between dropping out of a race for no reason because you are down in the polls, and dropping out because you just resigned your seat. Torricelli didn't resign, he was pushed out by the democrats.
Foley resigned. Of course, DeLay resigned as well, but that wasn't enough to get past the Texas law.
If they can't replace them, they can have a republican run hard to get people to vote for Pete Foley, after which because he won't serve they can have a special election.