Bill Murray is well loved, and rightly so. Although he's been in a fair few turkeys, he's cemented his reputation with solid-gold movies like Lost In Translation and Ghostbusters. There's constant speculation surrounding Ghostbusters 3 all due, it seems, to his participation. Apparently it's pretty advanced in development but may not go ahead if he's not up to it - and he himself has given lots of conflicting statements regarding it. One minute he says it's better to let it lie, and then he turns up at awards shows dressed in costume. There's probably a while to go before we hear anything concrete.
In the meantime however, he'll be taking on a biopic, playing American president Roosevelt. The film will be focused on his private life (he had an affair with a cousin!) and has also been in development for a while. As showblitz pointed out, it's been pretty hard to tie him down to the role seeing as he has no agent!