Work with a COPY of Photo Booth.app so that you can revert to the original behavior if you don't like the results.
Under Leopard, it's a simple matter of digging into Photo Booth.app and replacing the shutter sound-- /Applications/Photo Booth.app/Contents/Resources/photo_shutter.aiff -- with another short AIFF of the same name and in the same format (8 bit, mono, 8 KHz).
Replace photo_shutter.aiff with any sound you choose. You do have to give it the same name so that the program can find it.
Snow Leopard has tightened up code-signing. The above modification will probably break the app under SL, though it does work under Leopard.