"We can't wait to put Mailbox in the hands of everyone who wants it. This means not only continuing to scale the service, but also including support for more email providers and mobile devices. Add to that a host of new features and we've got a LOT of work to do... Rather than grow Mailbox on our own, we've decided to join forces with Dropbox and build it out together."
The post emphasizes that the app is not going away, but rather will receive the time, attention and development super power of a combined Mailbox/Dropbox team. Dropbox, of course, has the experience and the resources required to scale a service to support millions of users.
The app's long waiting list dissuaded some potential users, as did its support for a single email service. Reading between the lines, it sounds like the new Mailbox team will address both of these concerns in the near future. Our own TUAW team shared initial reactions to Mailbox here.
Best of luck to the Mailboxers and to their new 'box colleagues.