Here in the film industry, 64-bitness is helping with rendering performance and also on the desktop where artists need to use vast chunks of memory for their 3D and even 2D packages.
I recently moved over to Red Hat for Softimage. 64bit Softimage is a no go for me, the support and stability under Linux is useless, and it's just about bearable under vista 64bit! As you say for rendering, having debian 64bit dual boot on my core2duo box with mental ray works a treat! I have seen Maya used on 64bit boxes, with a quadro or two and like 8GB of ram, anything to keep that crap from crashing ;) Of course the Autodesk IFFS suites and the like are all 64bit, but I would rather buy a house.
Martyn, what 64bit 2D packages are you using?!? The closest thing I have to anything like that is a 64bit zbrush binary in windows :S
Rich