Some older film cameras were set up to handle over 300 volts, and some flashes had trigger voltages that high -- most digital cameras can only deal with about 6 volts. I've heard that different Pentax techs have been quoted from 6-30 volts for the DS, and mostly 6V for the K series bodies. From what I've heard, the high trigger voltages don't necessarily fry the camera circuits right away -- it can take a number of shots before the damage becomes apparent, so just because one tries it once, doesn't mean that it's safe.
The best source for trigger voltages on the net is:
If your flash guns aren't llisted, then there are instructions to measure it (or if you don't have a meter, try taking it to a Radio Shack or similar store).
Or you can get a trigger voltage limiter like the Wein Safe Synch.
With the cost of digital bodies, it's just not worth it to take the chance, IMO