Just did some reading about another activity based learning system called LAMS, developed at Macquarie Uni. Like Moodle, it seems to be based around social constructivist ideas - there’s lots of links to the IMS Learning Design specification.
Both LAMS and the IMS Learning Design specification have already been discussed on one of the moodle forums. Interesting to note that LAMS is a Java/Flash application, so quite different from Moodle (HTML/PHP). Also interesting to note that Moodle 2 is looking towards the IMS LD Spec!