Playing in random public places is NOT a good idea. I personally have had SWAT called on me before and that was all it took. Once they figured out what was going on, everything was fine, but still something you would rather avoid. In that instance, we were on private land and someone saw us from adjoining private land.
There are organized groups around the country that get together and play in areas that are safe from that kind of thing.
http://www.kcairsoft.com is a group near me. If you're interested in finding out more, or locating players/organized games in your area, check out:
http://www.airsoftplayers.com and
http://www.airsoftretreat.com
It is a great sport, but if people start getting busted in public w/ these weapons, it's only a matter of time before it will be illegal.
I've been in airsoft for a few years now, and would be happy to answer questions, etc. for anyone interested in the sport.