Hooray! Mailbox is now live in the App Store and reservations are being filled on a first-come, first served basis. (They’re using a reservation system to scale the Mailbox service gracefully.)
Currently for iPhone + Gmail, the app is available to download for free. If you’ve already reserved your Mailbox, download the app and enter your reservation info. If you don’t have a reservation, download the app to get your place in line. Everyone can watch the line move in real-time from inside the app.
I know a few very smart folks that have started using it and they all said it revolutionized the way they deal with email. Promising!