Ok – so this has been bugging me for a while.  One Vista machine on my SBS 2008 was not having mapped drives created via the Group Policy Preferences I had established.  I could log in with the same user account on another Vista machine as well as two XP machines, and the drives were mapped as expected.  I did some troubleshooting a while back, but didn’t spend a lot of time on it since it was an annoyance for me at best.

Well the condition has been fixed – thanks to a stellar find by Cliff Galiher in microsoft.public.windows.server.sbs:

After you turn on User Account Control in Windows Vista, programs may be unable to access some network locations
http://support.microsoft.com/kb/937624

I made the registry change indicated in that KB article, rebooted and voila! – my GPP drive mappings are working.