FYI this is a good walkthrough.
Letting your users “Sign In With Twitter” with OAuth and
How to quickly integrate with Twitter’s OAuth API using PHP. The latter post is a bit more technical.
Haven't looked at the Facebook Connect mod to see the level of difficulty of doing something like this, but it's definitely doable. I've PM'd sllik, the author of that mod to weigh in on this thread. Maybe it's something he's willing to tackle as it's so similar and a lot of his could could be re-used for this mod.