Last week Google Wave was open to around 6,000 developers and Google is planning to send out an additional 20,000 invites over the next month. In addition, Google is planning to release Wave to 100,000 users beginning on September 30th. So register your interest.
There is a lot of interest in this product - if you haven't taken a look at it yet - I suggest you do! I am registered to be informed when it is ready and can't wait to get my hands on it :-)