Alex,

We're (still, missed my upgrade window) on 4.0.2.1.

We've tested Vista with the 4.0.0.1 agent, and to the end user
experience, there are no problems.

On the Back end, our rules only apply to XP and 2000, so Vista will
get a free ride till Spring Break.  By free ride, I mean that no
requirements will be checked against Vista.

This is a concern for us, but we're not expecting a major damaging
worm the first few months.  We're also pretty confident that Windows
Update is automatically enabled on Vista. (We haven't really been able
to confirm this.)
As for saturation rates

We have 1100 of our 2300 average on right now.

I have a  PSP (shows as a *)
1 linux
29 OSX
1 Win95
1 Windows2000

No WindowsNT as I can see. (This is what I believe Vista will be
showing as, as well as Tablet and MCE)