ok, these arent exactly apples to apples, (nor precisely scientific - 2 machines and a stopwatch) but here's the scoop for anyone thinking of upgrading from a top end desktop g5 to a top end macbook pro :
macbook pro intel core2duo 2.5 17" 2 gigs ram, firewire800
g5 quad 2.5 4 gigs ram, firewire800
(dont know to what degree the 2x ram of the g5 had to do with results)
same project, same scratch settings, same version of FCP.
30 second tv spot, mix of SD and HD material, 7 composited layers
macbook pro: 3:15
g5 quad: 2:59
as a side note. same project on g5 quad
firewire800: 2:59
esata:2:48