I've been playing with piezo discs, they're fun but at $1 each I think that cap sense or conductive foam is cheaper.
I think of piezos as capacitors that you can change the charge through mechanical pressure. To that end, a piezo with a little resistance can be a kind of touch R-C circuit which is what I've made work and read using a digital pin and watching the time to drain from HIGH to LOW.
I saw one site linked to here, they use a piezo to vary a 2.5V current to sense the disc being blown upon. The circuit is good for sound detection which I'm more interested in. Seems to me that the circuit would also be good for zero-crossing detection. My own through-diode-and-transistor circuit is crap compared!
http://www.dtic.upf.edu/~jlozano/interfaces/blow_sensor.html
BTW, those force-strips are freaking expensive! I know there's alternatives including piezo discs. Check into how an electronic scale works some time.