Christine,
It is chucking it down here today, so I have spent some time trawling for something that will work on Canon camera's.
CanCount ( not free although there appears to be a trial version ) does not work on all models. I found a forum comment that says it is NOT compatible with 10D-20D or 300D -350D
Link to CanCount ......
http://www.soens.de/Software_EN.html
From another forum comment it appears Nikon added the shutter count to exif from the D70 but Canon only record this info on D1 series exifs.
I have looked at several threads on the subject on DPR - Canon forums, and from them, it appears that only a Canon Service Centre can accurately determine this for models other than 1D series.
Duncan, I have not found anything that relates to Pentax. Perhaps Pentax users are not too concerned so no posting or site links picked up by Google.
Don