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 isn’t quite as difficult as you may think, as you don’t 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.
There is a downside to this, as a virtualised PC normally uses emulated hardware – so the graphics card used to power Windows 7 will be nowhere near as good as a modern gaming card. This means you won’t be able to play recent games within the virtualised installation, but it is more than adequate for testing and playing with the new OS."