I think I just found a feature that's going to be implemented in Google Talk / Gmail real soon now: voicemail.
Try creating a label with the name "voicemail". I was cleaning up my personal account, where I get my voicemail delivered by email. I had set up a label months ago, and was trying to apply the label to one of the voicemails and got the error "The label name Voicemail is invalid". Hmmmm. So, I deleted the label, then tried to re-create it with the same name. No go...same error. Richard quickly verified that he was getting the same error.
Of course, trying to create a label with the name "Chats" gives you a different message -- "System specific names are not allowed. Please try another name." -- but it likely just means that "Voicemail" isn't truly live in the system yet.
Oh, and by the way, Bryght's switch to Gmail for domains is working really well.