Huh. That's a good question. The scenario being that you want to be notified when a new group with a specific tag is created?
I can't answer the "why" question... Once I understand the scenario better I can ask some of the designer folks. As to the "How", Ryan Rutan helped us out with some of the feeds from the community. He might know the answer.
The scenario is this: We've created one top level space for most of our different business units, we've then advised against using sub spaces, and recommended creating groups for their different functions, departments and regions, tagging them with something relevant.
I know there is a 'group list by tag widget' but I thought if we could have used an rss feed from the places results, then that could be more flexible.
Ideally, I'd like to have 'groups' included on a places tab in the community, driven by tags defined in the community settings. I guess that will be a bit of a customisation though.