Please see <dev.civicactions.net/?q=node/114> for my
  
initial write up of the "Taxonomy Cloud". This is something that is a
deliverable for CivicGroups and we will begin coding early next week.
Please comment at <dev.civicactions.net/?q=forum/34>
    

I get errors when I try to login with a Drupal ID.
This is a problem with CivicSpace I believe - this is just a vanilla CS install - I logged it as a bug over there.
I would recommend using "Easy News Topics" (ENT) [1] for this. It was
discussed (briefly) on Drupal.org [2] quite some time ago. This does
require RSS 2.0, but that is something that we need to implement in
any case. Paolo's been talking [3] about it lately again, too (it sort
of intersects with Technorati tags, but don't get me started on that).

[1] http://matt.blogs.it/specs/ENT/1.0/
[2] http://drupal.org/node/1655
[3] http://paolo.evectors.it/
I'm copying Anslem on this who is doing the programming.   We're pretty interested in sticking with RDF, but we're not religious :).