I help run a LANparty in Cape Town,South Africa. We cater for 600 people and this is a monthly event, and open to the public. First things first, right of attendance reserved, and enter at your own risk. Also, tell people to be aware of their surroundings, keep small valuables on your person at all times (wallets, cellphones, etc). Keep your case closed (not necessarily locked), and when in groups, try and get people to sleep in shifts, so someone is always awake to look over things.
As far as being public is concerned, yes it is, but those people who come to LAN have heard about it either on a local lanparty website or word of mouth, so that already narrows down the attendance "awareness" of people to those who are interested.
Yes, things like cellphones have gone missing, harddrives went missing, a handbag disappeared (yes GIRLS do attend our functions, and yes the bag was recovered) but in all cases it was due to negligence of the owners of said things.
Just make people aware of these things (use a PA system, or just a headset and BIG speakers) to let people know the houserules. Works for us, should work for you. YMMV on local laws though...
BTW it also helps that 2 of our regular attendees are cops ;)