Gah, merging accounts is a giant manual waste of time. I already have an account! Why can't I just associate it with my gmail account? I don't get it.
Mostly what Bambams said.
The old A.cc single sign on does not give me access to allegro.cc accounts in any way, other than sending the user to a.cc to login, it sends me back a token, which i then request the user information with.
Google's works similarly. With it's OAuth, you forward the user to Google's login, sends a token back to the site, which then uses that token to request user information from google.
The major problem is that Mediawiki's new authentication/authorisation framework changes how things work, and the GoogleLogin extension is a bit basic. It will allow a user to link an existing account to a Google user, BUT, and this is a BIG but... That requires the old auth method to work. It currently doesn't. The accauth extension needs to be finished, and it currently isn't. Nor do I care too much about doing it now.
I have no problem with manually merging accounts for people, and I gave sysop's and allegrodev group members on the wiki permission to do it as well. But I require anyone doing it has to make sure the user is who they say they are before merging accounts.