Now thatis committed and deployed on updates.drupal.org, the release history XML feed generates PHP warnings on the legacy crappy PHP4 XML parser in update.module in D6 core. We knew this was going to happen, but figured it was a lesser evil to the complication of a whole separate tree of release history XML files, especially given that official releases of D6 core don't ship with E_ALL enabled.
Luckily, it's a trivial patch that's already been well tested by both myself and mikey_p. Instead of it being lost and forgetten over at(which is probably never going to be backported to D6, anyway), I wanted to open a new issue for the patch:
Over a year later, it still applies cleanly to update.fetch.inc. ;)