1x4GB SODIMM RAM sticks just came out, and they're actually not that expensive ($179 @ NewEgg)
Read on a zdnet review site that a Rev3 or higher MacBook Pro would boot with 2x4GB sticks, but not work well
An enterprising forum member on some Australian tech site defied someone else's unsubstantiated data to show that in a 1x4GB+1x2GB SODIMM configuration it would work, so I ponied up to try it for myself along with a colleague.
Works just fine for me, second day running. Going to kit out the rest of the office with them.
VMs running with 2 CPUs and 3GB of RAM while your host machine is still humming along? Definitely worth $170...