P> Transmitter.
 P> Should be around 27MHz for legal reasons (no license rqd for low
 P> power). Range required no more that 10 meters. Absolute minimum parts,
 P> perhaps a simple colpits crystal osc? Need to be able to turn the
 P> transmitter On and Off via PIC port pin.

What about this idea:

              \|/                +------
               |     27.xxx MHz  | Any 20MHz-rated PIC
               +---+--|[]|--+----O in
                   |        |    |
                   +--------|----O out
                   |        |    |
                  ===      ===   +------
                   | 10p    | 10p
                   +--------+--------GND

You can turn oclillator off with SLEEP command and turn on with WDT.
All PIC I/O are free for some additional functions, if necessary.
27 MHz is slightly around PIC specification, however in practice it will works
without big problem with fundamental frequency quartz oscillators.

 P> Receiver.
 P> 27Mhz, 10meter - all I need is a carrier detection. Perhaps something
 P> similar to the old 'crystal' radio? (aerial coil, tuning cap, diode
 P> rectifier, low pass filter) followed by an op-amp comparitor. The
 P> output of this will be used to gate the ultrasonic sender.

Better use some of the one-chip fixed-frequency integral receivers with quartz
stabilization.

Alexey

Check http://www.geocities.com/SiliconValley/Way/5807 for some PIC ideas

--- GoldED/2 2.50+