People, start your engines — the public alpha is now available. Unfortunately, iCloud sync will not be available until it hits the Mac App Store — but then again no other apps supports the Tweetbot iCloud sync so that doesn't really matter.
Ironically, as I was about to hit CMD-Q for Twitter for Mac, it decided to crash. Way to go out with a bang Twitter app.
UPDATE: The app does support Tweet Marker, so I have temporarily switched my iOS apps to Tweet Marker as well.