Ratchet & Clank: Quest for Booty is for all intents and purposes the next game in the Lombax saga, and picks up where Future: Tools of Destruction ended. Gamespot says sort of the next game because Quest for Booty is at this stage being designed solely as a PSN bite-sized title to be sold for around US$15, and looks to be skipping retail shelves altogether. Sony reps pegged it somewhere around the three to four hour mark in terms of gameplay length, but from our time with it, its length is the only obvious concession being made.
This is the official spoiler warning for anyone that hasn't completed Ratchet & Clank Future: Tools of Destruction and would like to keep the ending a surprise. If you don't want to know what happens, skip ahead slightly.