I mean, sure, as a server Linux is exceptional. But as a client it is not even competitive in the business environment.
It isn't cost effective, time effective, or even well supported.
Many large companies have ended up with large installations because of a choose-your-own-laptop policy.
Ubuntu is easily the furthest any Linux vendor has come at producing a workable client, but there is still a long way to go.
Right now managing Ubuntu centrally is painful - I mean having it hook into Kerberos is easy enough but that only gives you control over users and remote files, you still have little control or visibility over the client OS its self.
I think Canonical offers a service for this.
Building a remote management solution that can mix the functionality of Active Directory and LDAP with Chef is one of the startup ideas I've been pondering lately.