> While we're talking about CCD cameras, does anyone know if any of the
> cheap cameras out there can be synced in any sense to an external signal?

Probably not easily.  You could use whatever "frame ready"
timing signal the camera generates as feedback to control
a variable oscilator which replaces the camera's clock.
The effort to retrofit this might exceed the work to build
the control circuitry from scratch.

> I want to combine several cameras as a cheap method of achieving high speed
> (slow motion) imaging, by having each camera take its frames in between the
> other cameras frames...

If you have 20 cameras, you could get 20 times the frame
rate.  But I assume the subject is in motion.  If so, the
images will probably be blurred since each camera's
integration time will be unchanged.  The image each CCD
builds up will be 20 "frames" worth.

You might be able to get around this if the camera module
has automatic exposure by using 20 times the light level.
But the camera module probably has a minimum integration
time (that I don't recall being a readily available spec
for the low end units).
                                                Lee Jones