Unless, the OP's question assumes that the Windows 7 installation is a full retail version, I think the answer is "it depends." Others may have a better answer. But my understanding is that if you purchase an "upgrade" for XP to 7, the installation process will invalidate or transfer the XP license to 7. If you purchase a full retail version of 7, obviously it would be possible to do a dual boot with XP.
I still think that until the actual discs are released or until MS starts answering some of these questions, this is all just speculation.
BTW, slightly, OT somewhere on another thread I thought someone said that you would be able to use the public Win 7 RC "key" as the "previous OS license" by which you could then "upgrade/clean install" the upgrade version of Windows 7. I thought that sounded bizarre and there was no link to a source. Does anyone know anything about this?