With both my Vaio SZ and TZ I wiped them clean on arrival and built them from the ground up with Vista Ultimate. It was a total nightmare getting all the drivers and some of the more useful 3rd party software reinstalled (e.g. WinDVD).
Now, finally, the Windows OEM's are delivering naked machines which is goodness for Vista. With this shift we'll see much more realistic Vista performance and behaviour for users. Hats off to Sony and hat tip to CrunchGear.