MacBook Pro here as well with two virtual Windows machines on Fusion. First machine is XP SP3 and is mostly used for Email (not a big fan of Entourage). This will probably go away with the release of OS X 10.6 (Snow Leopard) next year when Mail, iCal, and Address Book are all Exchange 2007 compatible.
My other virtualized machine is an NS running Windows 2003 and SQL 2005. Even with all this running OS X is still very useable.
Oh, and one other thing...I hardly ever reboot my Mac - just suspend the VM's and put 'er to sleep for the night...good Leopard!!
