I think he'll stay. While he may be exasperated with the film makers he has to know that if he drops out it would break some fans hearts.
What worries me is that according to some articles the White Council is going to be shown on screen. We know the Saruman, Galadriel, Cirdan, and Elrond were on it with Gandalf. Since five people would be pretty small for a council, it's been speculated about who else is on it. I've heard Gildor, Glorfindel, Erestor, Celeborn, Thranduil, Galdorn, and Radagast all speculated as possible members. While for most of those it would be no problem because they weren't in the first movies in any recognizable form (trading cards don't count) Saruman, Galadriel, Elrond, and Celeborn all represent problems.
Galadriel played by Cate Blanchett could still pull off the eternally youthful elf. So, unless the movies are delayed for another decade, it's not a big problem.
Celeborn: Okay, I had trouble believing Marton Csokas as an elf lord in the Fellowship of the Ring. I sincerly doubt that now that he is a decade older (he'll be 46 in 2012) I could believe him as an elf.
Elrond: Hugo Weaving is 50 this year. He looked too old for Elrond in Lord of the Rings. Now he would look way too old.
Saruman: Christopher Lee is 88. While he is still filming movies, there is going to come a time when he isn't able to film any more.
Quite frankly if they don't start filming soon they're going to have to find a completely new cast because all the original actors will be too old.