> "When I clicked on my name in the user-switching applet, I was immediately confused by the "Available/Busy" option. If I change that, what happens?"
The Empathy status is not the only thing that this switch will change: if you set it to busy, you will be busy not only for your IM contacts, but also for your whole desktop. And as you just told your desktop you are busy, it won't bother you any more with **any** notification, be they incoming chat messages, or updates availability, etc...
So it does make a lot of sense for the desktop to handle that as it is much more global than just IM. (and to be honest, it's a huge productivity boost in my case).
Note that "urgent" notifications will still be shown, for example "your laptop battery will run out in 5 minutes". :)
This is not yet implemented in Gnome 3.0, and I'm not even sure the design is finished. If you want to know about the plans, the following pages are a very interesting (although long) read:
ASIA (Adam says it all :P), but for a longer explanation about the design, see Jimmac's nlog post and specifically this comment where the final design was suggested:
About the multi-zone clock, it's planned, but not implemented yet:
Again, to add to what Adam already said, there is also this page that might not be completely outdated: