We starting rolling out Vista across our School a while ago and should be finished by the end of the Easter break.
The new version of RIS - WDS works great. We created two images one student, one teacher and then use PXE boot from clients to install the images to the PC's.
If you ou a whole room on it does slow it down a bit. One PC can take about 1 hour from start to finish but a room of 30 is about half a day and we have gigabit to the desktop. The images are quite large though as they have all our software preinstalled.
The images work as Microsoft say and one image will work on virtually any machine, the new .img file format is ttally hardware agnostic.