In addition to the announcement of 1 million reservations filled, the Mailbox team has announced that it has moved into its new office with Dropbox. And, maybe more importantly, a new update for Mailbox has passed through Apple's approval process and is live in the App Store.
The update offers "hundreds of fixes and improvements big and small", but the only addition that you're likely to really notice is a new "shake-to-undo" feature. So, you can now shake your iPhone to undo the last action you performed.
If you want to try out Mailbox, you'll still have to sign up with the reservation system, but if you already have it, just hop over to the iTunes App Store for the update.