The users/show API includes a "statuses_count" field which tells you how many tweets the user has posted
http://dev.twitter.com/doc/get/users/show -- 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
