One user account for all Wikimedia projects? How could that work? There are already name clashes, what could they do about that? Or could you just make one user account and have it automatically put on certain other projects, too? I don't get it.
There would be clashes, and the difficulties over how to resolve those are the main reason single login hasn't happened yet. For more details, please see http://meta.wikimedia.org/wiki/Single_login which lists a number of proposals and http://meta.wikimedia.org/wiki/Single_login_specifications which has the most recently discussed proposal. The Research Team are actively looking into this issue, and the results of their last meeting on it can be seen at http://meta.wikimedia.org/wiki/Wikimedia_Research_Network/Meetings/2005-06-18#Single_login
Angela.