userId bit has been there from the start in Universal Analytics. Its called custom dimensions and can be used to send any property about the user into the GA and then link it to a User or a specific Visit.
had missed that thanks for sharing. I have been using custom dimension for uId in our client projects its worked out well, though a dedicated api method is always welcome.
Custom variables don't let you consolidate on users though? In the visitor count, for instance, I don't think there's a way to tell GA to use a custom var to distinguish between visitors.
yes there isnt a way within GA to aggregate metrics based on custom dims at the moment. we have on occasion done it through reporting api using cd to maintain a unique count.
As of Jun 27 it hasn't shipped according to a GA team member.