"But a majority of medium and large enterprises have switched over to Active Directory because it's not possible to run a large Windows shop without it."
In the interests of avoiding redundancy I won't go into dirXML and NDS's hetrogeneous support and features previous posters have mentioned.
I'll assume you're talking about desktop management and populating the directory. Nope, that can't be it, ZenWorks from Novell does that and works with eDIR (Zen will run on Linux later this year).
I'm sorry, I'm confused. Not possible? I'm sorry but you're going to have to do better then that.