Some has beaten a giant, oh wait that is from a movie...but someone has installed Windows 7 on a UMPC.
Over at Ultra Mobile PC Tips they managed to squeeze Windows 7 onto a Amtek U560. This device is run by an Atom A100 running at 600MHz and with a mere 512MB or RAM.
They did have to use an external DVD-ROM drive but in the end had a working version of Windows 7.
Take a look