I don’t know if you’re familiar with Drupal (www.drupal.org), but its worth a proper look, particularly its Taxonomy system – every category term or combination of terms can generates its own RSS feed by default, providing an excellent base for advanced web2.0 functionality (like you’ve mentioned above) – I’ve often thought there should be a museum distribution of Drupal, its perfectly suited to the task, and seems way ahead of other open source projects of a similar type….