@Grace just for the record, do you know exactly what's happening to badges? I mean everyone probably keeps vote-recieved badges, but I'm not sure about vote-awarded badges (like Electorate). Exactly what data is transferred?

Eg: revision history, vote history, user login history, comments, etc

@Shog also

@Manishearth Users aren't migrated at all - you create an account on the destination site, and if something you wrote gets migrated over it gets associated with that account.

Your login history is actually separate from any one site - well, mostly at least. But for the purpose of the Enthusiast / Fanatic badge, your visits to each site are tracked and those badges are tied to a specific site.

Comments are attached to posts - they'll get migrated along with the posts, and re-associated with their authors assuming their authors created an account on the destination first

(you can always regain your posts by creating an account later, but not so for comments)

When a post is migrated, if its score is < 0 it is set to 0. Otherwise, it is preserved. Votes are migrated, after a fashion - they aren't necessarily associated with anyone, but their chronology is preserved: this is important for stuff like the daily rep cap, Mortarboard/Epic/Legendary badges, etc. And of course, their owner gets the proper amount of reputation on the destination site.

There are a few other subtleties to migration that probably aren't important here (you can end up with the same person voting twice, for instance). I'm probably missing something more important, but it's late - if I didn't answer your question, let me know.

@Manishearth Revision history is not migrated. Original author gets credit for the last revision post-migration. And bounties do not get migrated.