Pl. see this - http://en.wikipedia.org/wiki/Wikipedia:Wikipedia_Signpost/2008-03-24/Single_User_Login.
Of late, I've been more off Wikipedia than on it; so, I do not know how the community reacts to SUL. I personally feel that SUL is indeed a good system in non-controversial cases where there is no conflict of 2 users having the same username on different Wikimedia projects. It establishes one's identity clearly and completely even for projects on which we may not have been working and thus the uniqueness of our usernames is protected. I also believe that it is important that people who have gone for SUL signal that their username is unique on all Wikimedia projects - I created the template http://en.wikipedia.org/wiki/Template:User_unified_login to signal that. Pl. feel free to use it/ improve it or include it in various Indian language based Wikimedia projects.