Oh dear...the backwards compatibility on the 360 is very, very broken.
It seems that the only sure-fire way to play your Xbox games on your 360 is to keep your original Xbox. There are stories of the latest software update for the 360 leaving machines unable to play ANY original xbox games at all; as well as stories about games that are MEANT to work not working. One comment likens the Backwards Compatability updates for the 360 to Windows Service Packs and that very thought scares me.
Just read the thread, it's a bloody mess.
Xbox 360backwards compatability