"Unfortunately this collaboration doesn't exist in the blogosphere where tagging is limited to blog authors. We have added collaborative tagging to some intranet blogs and the results are interesting, but this would really bloom on the world wild web."
The thing is Paolo, collaborative post tagging is already here! It's just that Google are being really coy about it.
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. For instance here's a page (and corresponding feed) of the posts I've tagged 'mobile'.
Google Reader makes it simple and quick to add tags - shortcut keystroke 't' (for tag), then comma, then 'mobile', and Enter. Done. I've effectively become an information router down to the most granular level, i.e., Feed -> Post -> tag1, tag2.... tagn
Of course Google aren't yet exposing the full tag set assigned to each post but here's a bet they'll be doing so very shortly. It'll be a Technorati killer!