About collaborative blog tagging, EirePreneur says that it can already be done with Google Reader.

What I have found myself doing more and more of late is using the tag feature of Google Reader to create personalized tag feeds for some of my contacts and colleagues.

Quite interesting, I haven't seriously looked at Google Reader tagging yet. But this is not very collaborative: it allows you to recategorize other users' posts with your own tags, while I would like a Flickr kind of integration, where everybody (or maybe only some buddies) can tag any post and tags are visible on the original blog.

Anyway, Google is definitely another important player and it's interesting to notice that they are looking at tags, even if the fact that they don't support Technorati tags is quite dumb imho (Google Reader should recognize the tags that I add to my posts, while apparently it isn't).