Tags are being overloaded today as info-routers -- a loosely-coupled, loosely-typed way of shunting around data. People are excited by this, because it just works. For the record, I'm not against tags, I just have some concerns about how current implementations are susceptible to spam, and really am more interested in the next step beyond -- personal and shared tagspaces.
This is not quite topics in the sky or anything like that, but more of a pointer or agreement that a group has decided to mean the same thing when they're using tags. As well, an agreement that they've decided to populate/aggregate a particular tagspace (instead of everyone using someone else's tagspace). For example, What's Web 2.0? might create a glossary defining terms (== tags) used there, and I could point all the tags that I use locally at their tagspace (this last link will be up in a bit...).
I talked a little bit about the del.icio.us use of the "for:username" tag while explaining my use of links. That got me thinking, along with observing how Flock might eventually deal with groups, about blessing or formalizing the use of "for:" and perhaps "forgroup:" as standard usages.
There is background for this, in the use of tagging that acts as a review or rating (what's the link for this? rating:6 plus Amazon ASIN), as well as intense use of place name and/or latitude/longitude usage to tie together information such as photos or blog posts to map data, a.k.a. geotagging.
Permissions is the tricky part of this. People can spam me directly by adding "for:borismann" to their del.icio.us links. A group would have the same issue. But, if we can set those issues aside for the moment -- basically leaving it up to the implementing system on how to deal with this (e.g. friend and group lists that only allow certain accounts to do this) -- this might be an interesting way to form another level of organization around "just" simple tagging.
Finally, a ramble on "blessed" tags. It's a bit like a bottom up taxonomy creation and/or knowledge gardening -- watch tags form, then bless certain ones with special displays that give people an incentive to stick to the blessed formats of tags. For example, on 43 Places I stick to a personal format of making sure all eating places are tagged as "restaurant". Helper displays for metro areas would be quite easy to show -- e.g. top 10 most visited restaurants for Vancouver (teaser: and might actually be extracted remotely with a forthcoming API). Still, the spam issue. How to give people anti-tagging powers? Hey, I know -- we'll bless the "not:restaurant" format! A whole new level of tag fighting...