I believe most commercial plugs are written for the Maya 4 api, meaning 4.0, 4.01, 4.02, etc., but changed to 4.5 and they no longer work. I'm sure patches will be forthcoming from all companies,especially Pixar as RAT 5 hasn't been around too awfully long yet.
Darkon
Red bellows of flame have blackened my stones
Convulsing my frame and cracking my bones
Hell's dragons of steel who roar in their chains
Crawl into my caves to suck out my veins.....
-The Mountain P.F.M.