I once brewed a pale ale and decided to try gelatin to clear it. I added the gelatin to the bottling bucket right before bottling. When I cracked open a bottle a month later, the beer came out clear and delicious...except for the putrid-looking gelatinous clumps that had settled to the bottom.
The logical solution seems to be to add it at secondary, but I was concerned -- will using gelatin to clarify the beer inhibit carbonation if I bottle...i.e. will there be enough yeast still in suspension to promote the carbonation AFTER the gelatin has done its magic?