The mother of all yearly conventions I think is the Vegas Lifestyles convention usually in the first week or so of August. We've gone in '05 & '06 and it gets somewhere around 2500 couples. We skipped last summer at the Tuscany (it was always at the Stardust until it was torn down). We've always had a blast and plenty of hookups and after parties, and it's VEGAS! Though there's nothing posted yet probably because its only January, I'd imagine they'll start promoting it soon. Also a newcomer convention we've seen advertised is Swingfest in FL in July
http://www.swingfest.com although we have no idea if it will draw or not. The one time we signed up for Nawlin's in November the city got destoyed by Katrina so it was held in Vegas as well ; that was a lot of fun too.