I recognized today that one of the major by-designs of the ADUC Dial-In Tab is now in the public Knowledge Base:
837490 The Dial-in tab does not appear when you use the Active Directory Users and Computers snap-in to view the properties for a domain user on a Windows XP-based computer
This fixes the issue with XP-Clients that the Adminpak does not contain the Dial-In Tab. Those settings had to be done using Remote Desktop Publishing (RDP - formaly known as Terminal Services Administration Mode). Using the fix mentioned in the KB the Dial-In Tab is now available on XP Clients.
Other issues of my "favorite" tab:
You are not able to delegate rights - if you don't have full administrative access then clicking on the tab displays an error message "Could not load the Dial-in profile for this user because: Access is denied." and the page appears empty with just a message "Dial-in page initialization failed." This is fixed (see below - don't stop reading here) but not in the public KB, call PSS and request the following fix:
822542 Delegated administrator not able to open the dial-in tab
After you implemented that fix, delegated administrators are able to receive the tab, and enabled and disabled fields based on the delegated rights work.
However, there's still an issue: If you don't have delegated rights for the full Dial-In Tab, you are able to change the stuff you have rights to in the GUI, but it won't take the changes when you hit Apply or OK. My issue is that a customer wanted the msRadiusCallbackNumber delegated - their helpdesk should not be allowed to decide whether or not a user had Dial-In Rights, but to change the Callback-Number if needed. We have a open case on this, if anyone else needs delegated permissions within this tab (and has preferably a support contract with MS) let me know - as far as I know right now this will not be fixed for a single customer.