UMPC Fever has a forum member who installed Windows 7 on a Fujitsu U2010, which worked great, from the included screenshots. They were able to use Vista drivers, and it was supported better than XP. They mentioned it was faster than Vista.
Ultramobile PC Tips had news of another forum user on a Spanish-language site who had installed Windows 7 on a Gigabyte UMPC U60. They reported that it booted faster than Windows XP Tablet PC Edition (SP3) and runs just about as fast overall. It also had no problems with the U60’s WiFi (Vista did). (via SlashGear)