Hey, I think this is a grrrreaaat idea. So when I hit my 4000th can I have a new car?
Just kidding (about the new car). But it is a great idea. The folks that I dive with usually have a little celebration for someone who goes over a "milestone". Usually it invloves a grilling some food, drinking a few adult beverages and baking a cake for them. There are usually other mementos given as well, in recognition of the accomplishment.
Perhaps a DiveBuddy tee shirt or even a DiveBuddy coffee mug with the number of dives printed on it would be appripriate.