I have yet to work in an enterprise environment that used Ubuntu or any Debian system, I am a Linux administrator and yet 90% of the jobs I come across are administering Active Directory and Windows Domains. Bloomberg uses Windows, The Navy and Marines just signed a 700 million dollar deal with Microsoft: http://www.informationweek.com/government/enterprise-architecture/navy-microsoft-sign-700-million-agreemen/240003281
I remember when I was training to become a Linux administrator my instructors explained that most corporations that use Linux Only use RedHat(RHEL) and Suse(SLES). I hear alot of noise about Ubuntu, but rarely do I see large corporations utilizing Ubuntu Server where I live. I personally think Debian is easier to use than Red-Hat, but have yet to actually have to do much professional work on Debian systems. I would love it if Linux was used more, but the fact is that Microsoft's Active Directory is still the leader by far when it comes Enterprise Directory Services. Server 2012 will only add to this domination as it is making a strong push in the VDI game.