Mary Jo Foley reminded me this morning that the Messaging app in Windows Phone 8 does not support group messaging on Verizon. Apparently, the wireless carrier is able to disable this feature, presumably for spam reasons. I'm not sure yet if this affects messaging to Groups or Rooms, but suspect it at least affects the former. I will try to find out and need to add a note to that effect to this chapter.
Of course, this raises a broader issue around which Windows Phone 8 features can be disabled by carriers. I doubt Microsoft will want to address that, and suspect such a list would be longer than wanted and somewhat polarizing.