A number of sites do this directly through stored procedures we expose in SQL for adding, updating and deleting subscribers - the synchronization with AD/Exchange is done automatically for you when using these stored procs.
They aren't TAC supported yet (Connection will be the first release that supports limited provisioning interfaces) but they are the same interfaces. I have code samples and documentation on how to do just this type of thing out on www.CiscoUnityTools.com if you're interested in taking a look.
I've worked with several fairly large installs that do their own provisioning hooks using this and so far no unsatisfied customers other than a couple that really wanted a SOAP interface to work through (comming in Connection).
