U.S. Supreme Court
This is a slim possability if Ruth Bader-Ginsberg decides to retire anytime soon, which is not likely. However, if she does, the Senate tends not to attack one of their own and he would probably be a relatively safe choice for Obama.
Job Somewhere in Obama's Administration
I would love to see this-- especially something like U.S. Trade Ambassador. But, realistically, Obama has shown that he is a corporatist and Feingold's resume is probably the last one they consider. The only way this would happen, though, is by some serious arm-twisting by frustrated unions demanding some love, which is a possability.
Run for Herb Kohl's Seat -- If Kohl Retires
This seems like the most likely scenario. Althouhg Kohl is in great health, he's pushing 80 and I just can't see him wanting to go through the headache of what will likely be serious opposition in 2012 and then serving out another term that would tie him up until his late 80s.