I can't seem to find any details on it on their site (it's absent from Features and Documentation) and nothing relevant came up in a quick Google search.
It's there. When viewing your project's details, you've got a tab called "Downloads". You can then upload stuff like binaries, and I quote from that interface: "Add any file that you would like to make available to your users, such as app binaries"
Ah. I don't have an account. So, to me, there's nothing there at all. And if it's not documented (not even a mention in features or the documentation) other than just existing in the admin interface, I don't know that I'd trust it to be there next week.