I think what Vista is really doing with the flash drive is putting the Page File onto it. Basically using a flash-based hard drive to use as virtual memory. If your flash drive is fast enough, you will see a performance boost over, say, an Ultra100 IDE drive, especially if said drive is the only drive in the system. The benefits in this situation decline as SATA/SAS and RAID setups are introduced, especially when a set is dedicated for the swap file. (That last bit is an extreme example, as the smallest SATA drive I know for certain exists is 40GB)
Then again, I haven't really looked into what Vista does, but this makes sense to me.
(Goes to dig through the topics about the server...)
