You can efficiently determine which lists owned by the authenticating user have a target user as a member by adding the parameter "filter_to_owned_lists=true" to :user/lists/memberships.
This should probably be included in the documentation here: http://dev.twitter.com/doc/get/:user/lists/memberships -- Twitter developer documentation and resources: http://dev.twitter.com/doc API updates via Twitter: http://twitter.com/twitterapi Issues/Enhancements Tracker: http://code.google.com/p/twitter-api/issues/list Change your membership to this group: http://groups.google.com/group/twitter-development-talk
