Someone just referred me to this yesterday. The Easy News Topics RSS 2.0 Module might be potentially useful for connecting Drupal sites. Unlike XFML which requires a separate feed, this is a way of expanding RSS 2.0 to output Drupal terms for each item. But as it's an RSS module, it's ignored by anyone receiving the feed that doesn't support the standard.

Like XFML, Easy News Topics has an element for mapping classification schemas using a topic map. Obviously useful for Drupal sites to communicate whether they share the same vocabulary or not, thus allowing incoming items to be classified by feed supplied terms, rather than terms applied to the news feed itself by the receiver. Potentially, Drupal communities could have shared XTM's which community members could download and import into their taxonomy as a new vocabulary, making it much easier to begin exporting term tagged items to a community site.

While I guess I could have entered this as a feature request, I thought it might be equally interesting conceptually for the work that I read about which developers are currently implementing with RSS and the import module.