I just read an article titled "Virtual Directories Take Hold" that was published in the April 16, 2007 issue of Network Computing. Interesting to note that the author states it is "not surprising that Microsoft and Novell provide virtual directories".
I'd like to find Microsoft's. As far as I know it is still - at best - a virtual idea...
p.s. I'm still not a big believer of these things.
Microsoft, Active Directory