I wanted to make custom attributes accessible from the Users and Computers MMC. The Microsoft acctinfo.dll adds an 'Additional Account Info' tab, so I knew it must be possible.
Unfortunately adding tabs turned out to be complicated, but found instead I could add right click options very easily. Sites I came across all used VBScript, but you can use just about anything, which allows for creating a proper GUI.
So now in the downloads is my VB6 example project, with screenshot to help explain. http://www.greycube.com/site/download.php?view.61
This opens up a lot of possibilities, at work the option not only show custom A/D stuff, but also links to other systems, and runs powershell to replace tabs removed in Exchange 2007.