On the PC, I've used a program called "Power Data Recovery". I don't know if there is a Mac version, but I highly recommend this program for lost data recovery.
Note: If the pictures were simply deleted, DON'T do anything else to the card prior to using a recovery program. The information is actually still there and perfectly intact. All that has happened is that there is a part of the file structure that points to where the data actually is that has been deleted. As long as you don't write any more data to the card (take any more pictures), you won't mess up the data that is there. The recovery program should have an easy time getting you pictures back.
Good luck.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70