I think the main reason there's not an emu for the original X-Box is the lack of exclusive titles. Everything worthwhile was already on PS2 or PC. I think the guys that program emulators in their spare time looked at it and thought "our time would be better spent trying to perfect Gamecube and PS2 emulation instead".
As for 360 and PS3 emulation, wait 10 years, if it ever does happen. But I see the 360 being emulated long before the PS3 ever does. And even then, it will probably just be the DLC games since they would be far less taxing on resources than a disc based title like FF 13 or SF 4 could ever be.