I have read that they might spawn for you, but I have also read that eggs and the fry that will hatch from them are really tasty thiings to eat for your other fish. There are a lot of pages and resources for mating your clowns and raising the babies, but if you have anything else in the tank with them I think they will just be producing some natural fish food for your other tank residents.
Most of the time when purchasing mated clowns, its so that you can have a pair that will interract in a friendly relationship with each other, instead of fighting with each other till one dies.
There is a smallish book called "Clownfish" that I haven't bought yet, but I flipped through it at the lfs and it has a lot of information in it about breeding clowns, and I know there is an abundance of other books on the subject. Maybe someone with one in their library can suggest a good one for you if breeding is an interest. As
I just bought some clowns for my gf for valentines day I will probably be shopping for some books for her, and I'll post about any good ones I pick up when I'm done reading them.