I've got the new February CTP build of Vista and I am in the process of installing it on the M400 that Toshiba NZ have loaned me.
The first thing I noticed is that this build is the first to offer an upgrade option (from Windows XP SP2 anyway). I've opted to do a clean install in the first instance, though I may restore factory defaults again and try an upgrade after that - now that I have the unit unitl Monday.