Microsoft's just thrown down a brand new Xbox 360 backward compatibility update that adds or updates about 40 titles (give or take) to the list of Xbox games it can play. If you remember back to GDC, you'll know that Microsft's planning on ramping down the BC efforts after '07. So unless they pull out a gigantic update near the end of this year, you're probably going to have to keep that original Xbox around if you've got some titles you're still keen on playing.
That said, it's nice that they finally got some of the old Call of Duty games up in there. You can never get enough Nazi killing.
April '07 Back Compat Update [Major Nelson]