I have a whole bunch of invitations to App.net — you can sign up here.
If you don’t know much about App.net, check out the weblog, where they promote third-party apps, link to their podcast, and talk about the APIs they’re building. It would be wrong to call it a Twitter alternative — it’s very much its own thing, and worth checking out.
I’m a fan. Love the vibe. I’m @brentsimmons there.