At the end of the GDC blogger's breakfast yesterday morning, Chris Satchell dropped this bomb that not many people seem to have picked up on. He said, regarding Xbox 360 Backward Compatibility:
At some point we're going to focus less on it. When you get to the end of this year there will be a reduced focus on backwards compatibility. There are so many 360 games out there. I don't know if it's important anymore.
So depending on how many Backward Compat. updates (we counted two in 2006) your Xbox game may not quite make it to Xbox 360, which means you're going to have to keep that big black box around indefinitely.