W7Forums have a new article on how to run the Windows 7 beta within your existing Vista/XP installation using VirtualBox, eliminating the need to dual boot or perform a clean install

Trying out Windows 7 isnt quite as difficult as you may think, as you dont need to wipe or dual boot your existing Vista/XP installation to try out the new Microsoft operating system. It is entirely possible to run Windows 7 from within your existing Windows installation, at the cost of reduced performance. All of this is thanks to virtualisation technologies which allow you to run a computer within a computer.

How to Install Windows 7 within your existing OS