I really like the idea of having one VM for many languages, but having just watched the Google IO video on V8, I think there might be some real performance advantages to specialized engines.
An specialized VM (both for the source language and the deployment architecture) would leave anything generic in the dust. Languages do not have the same usage; the truly unique ones (i.e. worth learning) encourge different programming styles. The G-Machine is as different from the Warren Machine is as different from a SECD machine and all are different from P-CODE machine or an SKI graph reduction machine.
Smilarly, a custom made application DSP would rape any other binary for a stock hardware platform of the same speed (ignoring all costs, of course.)