I think XEN is what is scaring the piss out of Parallels right now. Apple has a SOLID history of putting open sourced features into their OS. While ZEN wouldn't == game over it would put a definite dampener on Parallel sales. Not sure about VMware if they make the client really cheap or free. Again for me VMware's biggest feature is cross compatibility.
I don't think Parallels is emulating a vid card. What I think is happening is what happened in the days of predual core CPU's. Two OS's sharing a single GPU is obviously going to run slower then if the virtualized envir had its own discreet GPU. Also I think Parallels is pulling a fast one with the games demoed.
As an example Quake that was being demoed. My money is on parallels doing some form of pass through for Open GL calls which would be a heck of a lot faster then using Direct X. Again IMHO Parallels is playing games with their customers.
I just finished backing up my Parallels VHD and removing Parallels. I'm going to give Fusion a shot because at this point Parallels really has irked me enough to try the other white meat.