Well Microsoft has finally announced the backwards compatibility list for the 360. The list contains more than 200 games (a lot more than I expected), and they say more may be added before the 360's November 22nd launch.
According to the Q & A with Todd Holmdahl it also points out this very interesting fact.
One of the great things about gaming on Xbox 360 is the satisfaction of knowing that every game will be playable in high definition. We are now proud to reveal that this extends to the original Xbox games as well. Every original Xbox game will be upscaled to 720p and 1080i, and will take advantage of Xbox 360Âs anti-aliasing capabilities, delivering a picture that is clearer and crisper than anything available on Xbox.