it's worth mentioning that, once the functionality exists, people will start asking for autocomplete. (Although I guess they haven't asked for it in the markdown? So possibly I am borrowing trouble.) As long as you restrict autocomplete to people in their circles, and do the messy stuff to make autocomplete accessible, shouldn't be too load heavy, if that comes up.

...I actually have no idea how sites with a userbase like Twitter scale the autocomplete of usernames function. It's not particularly slow and is pretty accurate even if you are not logged in and it doesn't have a FOAF circle to go from, so I have no clue.