Why don't you change the date and make it around the same time as DEF CON 19? That's the first week of August. There should be a lot of people interested in Bitcoin, and I believe a presentation on Bitcoin as well.
http://www.defcon.org - Rio Hotel, Las Vegas - Aug 4-7, 2011
EDIT: One thing of interest on the DEF CON 19 calendar: "Black Ops of TCP/IP 2011:
(among other things) BitCoin: Network Manipulation for Fun And (Literal) Profit"