A PS2 emulator for the X-Box is impossible.. well, not impossible, but it would be completely unplayable.. Sure, the Box's hardware is way better than the PS2's, but it doesn't have enough of a lead to make PS2 emulation worthwhile.. you wouldn't see the framerates go past 5...
Once linux is up and running on the box, I wouldn't be surprised if a PS1 emulator came out.. but don't hold your breath..