The simple delete command on most cameras just marks the space on a memory card as being available for use rather than overwriting the actual space on the card. As long as you don't record anything new onto the card, previously deleted photos should be relatively easy to recover using a file recovery utility.
You should even be able to recover photos from a reformatted memory card as long as you used a standard format command rather than a low-level format. (A low-level format not only deletes all the data on the card, but it also recreates the directories and folder structure of the card, rendering previously deleted files unrecoverable.)