After MacNN published the news about Mailplane, all available private beta seats were taken in about one hour! The website is still under heavy load, and the waiting list gets filled quite fast.
This is great news because it shows the need for Gmail Mac desktop integration!
For all those who didn't make it into the private beta: Add yourself to the beta waiting list. As soon as Mailplane gets more stable, people on the waiting list will get invited to the beta.