I'll be hosting an activity streams meetup tomorrow night in San Francisco at Citizen Space:
The goal of this effort is to start documenting representing activities and activity streams in the wild, and to collect the various classes of actors, actions/verbs and social objects in the wild.
There are already a few implementations that rely on the notion of the "activity stream" (plugins are available for WordPress and Moveable Type already) but the hope is to extract common practices into a format that can be used to enable services to add support for new types or services without (ideally) having to manually support each new activity provider as it comes online.
Our solution space is likely to be some combination of ATOM and microformats, but it's too soon to think about specific implementation details just yet.
I've got room for about 15 folks tomorrow night, so if you can make it, feel free to stop by around 7.