Responding to the stories that Microsoft's next generation Xbox 360 won't be as backwards compatible as the announcements on last week's MTV special and at games industry 'do E3 suggested, Larry Hryb, the Xbox Live Director of Programming, has posted an explanation on his blog:
We're not sure where this came from, and is incorrect. At launch, Xbox 360 will be backward compatible with the top Xbox games. Our goal is to have every Xbox game work on Xbox 360. You will NOT need to purchase a new 'version' — your original games will work on Xbox 360.
According to insiders, apparently how it really works is that rather than have a tiny Xbox inside (which is how the PS2 plays PS1 games), Xbox360 will run an emulator which emulates Xbox. This does however mean that some games, which do funny things with Xbox hardware, might catch the emulator out and so not work. Microsoft will try their hardest to make sure this doesn't happen, at least with high profile games.
cheers M!