As mentioned by Elizabeth Naramore on the php|architect website, this year's Joomla!Day USA will be happening next month on October 13, 2007 in New York City:

The event is open to Joomla! fanatics or anyone who wants to learn more about the popular CMS (both using it and coding for it). It will run from 9:30 am - 5:00 pm ET.

There'll be both the regular conference and an un-conference sort of structure to the day allowing for times for official speakers and any audience members to present their ideas (audience speakers will still need to register a topic ahead of time, though). You can check out the official schedule on their site and get more information about the wheres and whens for the day.