Revisiting this question - the best thing that you can do for this is to run your VM off of a solid-state drive.
Adding an SSD to my system (OCZ Vertex 3) dropped compilation times by 80%. The problem is that Visual Studio is very intense in disk accesses, and when running a VM, this is a big problem (since the VM is just one giant file).
Looking at the disk queues in resource monitor, the difference was apparent. With a 7200 RPM drive, the queue was stacked at over 300 queued requests during compilations. With an SSD, the queue never went above 5 requests.
Would highly recommend an SSD for anyone else in this situation.