I think the reason OpenID isn't used is just because the cost to implement it is fairly high (since providers are so bad at sticking to the standard) and the return is relatively low since very few people realize that they can use it. ;)
Since Wikia has significantly different user-accounts (because the accounts span all of the wikis), the MediaWiki extension wouldn't work out of the box, but it would definitely be a good starting point if we go down that path... thanks for the tip!
It would be cool to have OpenID, but I haven't heard many people pushing for it (and after having implemented it at a previous startup, I'm not to anxious to subject myself to that again).
It's great to see users blogging their ideas though :) It's a good way to get more ideas circulating around & hopefully get some of them to gain momentum.
Thanks!