MameCE already runs on the 3955. However it is very slow and unplayable when sound is enabled. With sound disabled Mario Bros and Donkey Kong Jr were running full speed. Those were the only games I tested.
One thing you should know about the Xscale processor is that no software (that I know of) is optimized for it. Sure you can run ARM code, but its not going to run any faster then the 206MHz ARM chip on other PDAs. There are even cases the it will run slower on the 400MHz Xscale.
> Does (or will) MameCE3 support the new IPAQs (39xx) that have the 400 MHz Intel