On the Canon 20Ddigital SLRcamera, the shutter actuation count is stored in as a 16-bit integer in bytes 0x95d,0x95e (big-endian). I've written a short program (linked from my write-up) that will take a list of Canon RAW files as arguments and return the shutter actuation for each in turn. The Canon 20D is rated at about 100k shutter actuations over its lifetime.