It seems there will be at least one more Mac OS X 10.5 (Leopard) update before we see Mac OS X 10.6 (Snow Leopard) arrive this fall. Apple has begun seeding Mac OS X 10.5.8 (9L14) to Mac developers.
Apple Starts Seeding Mac OS X 10.5.8 (9L14)
Apple Starts Seeding Mac OS X 10.5.8 (9L14)