Marc,

Lot of good ideas so far but I'll throw in my 2 cents worth as well.

I have used a piezo element driven directly from a PIC16C63 by two
port pins.  The PIC has good sink/source drive specs so I think
extra buffering is a waste of time with just a 5V supply.  The port
lines are toggled alternately to give maximum possible AC drive,
and yes resonant frequency is important to get maximum volume.

To turn the sound off just stop toggling and take both lines low or
high.  You could leave one line low and one line high but some
piezo manufacturers say DC for extended periods can damage or
degrade the device.

I also found that "warbling" around the resonant freq made the tone
somehow more noticeable, especially when there was background
noise.  A monotone also seemed to have dead spots in volume as I
moved around in the room.  I could try and get really scientific
about this, but perhaps the dead spots equated to dead brain cells
located between my two ears.

Note: with current exchange rate 2 NZ cents equals 1 US cent.

Brent Brown
Electronic Design Solutions &
HI-TECH Software Reseller
16 English Street
Hamilton, New Zealand
Toll Free: 0508 HITECH (0508 448 324)
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