Multithreaded Apps?

well just being multithreaded isn't enough, most apps are multi threaded. There might be a few threads to handle the UserInterface and various other things but most of the work is done in a single thread, which can't be divided between multiple processors.

but this isn't the case for all apps, say compressor 3, can use all 8 of the MacPro's cores.

 

Yeah, multithreaded isn't a sufficient criteria. There are many types of threads, only some of which can be spun off to separate cores.

I'm not sure about iPhoto, but iTunes, iMovie, and iDVD all use as many cores as you have. iWeb probably isn't, since it wouldn't see much of a speedup. Most "Pro" apps like Photoshop, FinalCut, Maya and Compressor are also multi-CPU aware.

There have been multi-CPU Macs for a decade; the dual G4s were very popular and lots of applications supported multiple cores.