The Joomla! Project is tentatively planning a developer conference that will take place in New York City this December:
Experienced Joomla! developers, go to this site to indicate your interest in this conference. This conference--and possible future conferences--will only be held if the Joomla! community expresses interest in it. And you can't beat the cost (tentatively planned for less than $100/ticket). There's a form at the above URL that you'll need to complete to let them know that you're planning to attend.