According to (the usually pretty reliable) Hoopsworld [http://www.hoopsworld.com/article_4312.shtml] Bremer is a restricted Free Agent (like Arenas, Terry, Hamilton, 1/2 the Clippers), so there would be no option. However the Celtics' site & ESPN don't list him as a FA at all. Patrica Bender's site says the C's have an option next year [http://www.dfw.net/~patricia/contracts]
I'm guessing then that he's not a FA till next summer -- restricted or un.