Rationale
From the user menu, you can currently switch from available to offline, which is useful, but I feel it isn't enough. If we added switching between available, busy, invisible, and offline, the menu would become more functional. Switching text status could also be a handy feature. Here's a screenshot of what it might look like.
Mockup
I got the status icons from fs-icons-ubuntu, available on deviantart and licensed under GPL. I'm completely open to using other icons, so please leave any suggestions or other feedback in the comments.