FCP indeed uses all my 8 cores... just spreads out the rendering tasks and uses them all however not to their limit. This leaves overhead to compress in Compressor, render in Motion all at the same time...
Check out
http://www.barefeats.com Rob there explains how all 8 cores are working in Tiger. I'd not be surprised to see improvements in Leopard, but gotta say, I've done routines that would have been a ton slower on fewer cores. I'd say that if you don't do multiple things at the same time the 8 core isn't for you though. Try using Compressor with a virtual cluster running with 8 cores. It's almost twice as fast as a 4 core machine, It pins all 8 cores to the limit according to the Activity Monitor.
Jerry