I have a SBS2003 server. I recently noticed that new security groups added to the server require a reboot of the server before their NTFS permissions work. Example:
I create a share called "Share". The share permissions are set for Everyone - full control. I set the NTFS permissions to allow full control of the folder to the "Group" group. I add members to the group and attempt to access the folder via an account that I added as a member. I get access denied. If I look at the effective permissions, it tells me that the user account has full access. A reboot of the server seems to fix the issue.
This seems oddly similar the a bug in SBS2003 where the RUS was not working and newly created mailboxes would not be available until a reboot. SP1 fixed that issue if I recall...