Steve provided a patch for this:

— confluence-orig/confluence/src/java/com/atlassian/confluence/rss/DefaultFeedBuilder.java 2006-08-28 15:35:20.000000000 +1000
+++ confluence/confluence/src/java/com/atlassian/confluence/rss/DefaultFeedBuilder.java 2006-10-09 10:59:32.000000000 +1000
@@ -71,6 +71,11 @@
entry.setAuthor(getUserFullName(entity.getLastModifierName()));
entry.setPublishedDate(entity.getLastModificationDate());

+ // This is site-unique, but not global. We could add the
+ // server domain here.
+ String uri = "urn:atlassian.com:" + entity.getId();
+ entry.setUri(uri);
+
entry.setCategories(renderer.getCategories(item));
entries.add(entry);
}

We need to change it slightly to use (preferably) the server ID or (alternatively) the server base URL instead of "atlassian.com".

I'd suggest a URI of 'urn:atlassian:confluence:<server-id>:<entity-id>'.