You need to authenticate with minecraft.net in order to play multiplayer. Sometimes this will go down, and hence you won't be able to play on the prom server.
Minecraft beta multiplayer support is currently under heavy development, and is riddled with bugs. You can help test it, though. It will get better soon, I promise!
I believe that's only for the login. I don't think the update and the mysterious call it makes to minecraft.net whenever you connect to a multiplayer server have been migrated (yet).