I've often wondered why Vista never seemed to work very well when banging in a UNC share at the Run box; it always seemed to be hit and miss. Server 2008 suffers from this also.
It seems there is now a hotfix for this problem.
Quite honestly, it never occurred to me to be a Vista problem, I assumed it was a network issue but couldn't really narrow it down. I was just living with it.
After applying this hotfix and rebooting you can now resolve UNC (SMB) shares lightning fast (just like in XP) and the way it should be.
This hotfix is included with SP2 when it arrives.