Hello Oded. I use wireless contact mics in a couple of projects. Both requiring freedom of movement for the object to be miked up. (See the links below)
http://www.myspace.com/entretenidos http://www.cryingoutloud.org/event_stretch.php
I make my own pickups (piezo ceramic discs) and pre-amps: I've often used this schematic which gives me a good result. http://scotthelmke.com/Mint-box-buffer.html I plug the signal from my pre-amp into the sennheiser freeport radio mic system which is good value for money, but limited to 4 transmitter/receivers at once which is more than enough for me. (it has a setting to alter the gain of the transmitter for instrument or microphone) The connection is conveniently based on mini-jacks. Did you have a specific problem or question I could maybe help you with? Nick.