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