I did try mapping a drive and no dice. Also, the script works on XP with me running it so permissions are correct. I'm sure its purely Vista. I had another guy on Vista try this and he reproduced the problem. I'm running this as a domain admin and I get the following message using Process Monitor:

28547 8:46:25.4002811 AM reg.exe 4832 RegLoadKey HKLM\test ACCESS DENIED Hive Path: UNC\Domain\Share\profiles\User\NTUSER.DAT
32293 8:46:26.0527129 AM reg.exe 4832 QuerySecurityFile \\Domain\Share\Profiles\User\NTUSER.DAT ACCESS DENIED Information: DACL

These are the only two access denied messages I get when performing this operation. Again this works when the file is copied locally or when the file is accessed from an UNC path on XP. Can anyone shed light as to why this is happening?