No new Xbox until 2006
Microsoft CEO Steve Ballmer has revealed that the next Xbox console won't be launched until 2006. "Next," in this case, probably refers to a next generation console whose capabilities would significantly exceed those of the current Xbox.
Even if the Xbox architecture remains unchanged until 2006, I'd still expect Microsoft to tweak the Xbox's design to further cut costs, shrink the form factor, and perhaps even integrate PVR capabilities to compete with Sony's upcoming PSX. Xbox hardware will probably see a few subtle changes to break compatibility with popular mod chips, too.