Perhaps Linux is used ALOT more than you think, you're just not aware of the installations ;)
I know of at least 2 places which are very large and influential organizations that run ALOT of Linux and other Open-Source Systems - in one of the organizations I'm thinking of I implemented Linux in combination with MRTG, PHP and MYSQL for an application I wrote for the purposes of systems monitoring and server inventory, something I whipped up because Tivoli, a large, expensive "enterprise" product was proving too cumbersome and taking too long to implement and my Management needed something RealSoonNow(tm) to do the job.
Unfortunately though, Non-Disclosure, and fear of being publicly identified prevents me from citing the organization(s) by name.
Linux is used in quite a number of places, but it doesn't get the big "The Department of xyz for the pqr Government is installing Linux" publicity.
Don't despair, Linux is making waves, you just can't see the ripples ;)
Oh and Linux has its own Directory functionality, it's OpenLDAP. It's just not necessarily as easy to maintain as Open/Active Directory.
No offense intended... but I did say that in my original post ;)