SI, it hasn't really been a problem for us. Most of the time the police are more interested in what airsoft is, and want to try out some of our gear, and I drive around with some pretty serious hardware. For most games I have nothing less than 2 M4A1s, 1 G36c, 1 sig552, 2 ak-74s, 1 M249 SAW, 2 mp5ks, 2 Beretta M9s, 1 M1911, 6 M67 Frag grenades, 6 flash bang grenades, 2 claymores, 40 or so magazines, 3 tac-vests, and of course, the bomb prop. For games we always notify state and local police (and campus police when using our University field), and we often have members of law enforcement playing in the games with us.
Electrical Juggernaut, this is the keypad I used for the first design:http://www.sparkfun.com/commerce/product_info.php?products_id=8653
I used a 4x4 keypad in later designs that I got at a local shop, it's the same setup, just 1 extra pin.
And this is the serial LCD that I used:http://www.sparkfun.com/commerce/product_info.php?products_id=9068