I've seen this too and I know why. Its a performance issue. Its not just the server that uses SIDs like that. 2000 professional also uses SIDs. If you wait awhile and refresh your screen, you can clear up alot of that (sometimes it takes a little time to find the username or groupname associated with the SID). My test server is using an overworked celeron 300mhz and it can take a few minutes to clear it up... especially if the user or group name was created on another computer (having to associate the username or groupname to a SID stored on a computer across my slow 10mb test network). Just relax and let the computer think for a few minutes. If your computer can't find the user or group name after a few minutes, that doesn't mean the user or group doesn't exist. It just means it can't find the place where the SID was created to associate a name to it. This will happen to me when the workstation is turned on and the server is turned off. I log in locally to the workstation and the shares can't find the domain group names on my workstation shares (only shows the SIDs because it can't find the group name assoicated with the SID on the server). When I turn the server on and wait a few minutes, it clears up and shows me the domain groups I'm sharing to. The SID itself will give you a clue to which computer it was created on. It's two parts with the first being a unique id number associated with the computer it was created on and the second part associated with the user or group created on that computer.
You only discover these things when your network is faster than a speeding turtle.