This is my thinking also to which I broke down how the books will be divided, especially since the remainder of the Power Books come after PHBII.
Arcane will cover Bard, Sorcerer, Warlock, and Wizard
Divine will cover Avenger, Cleric, Invoker, and Paladin
Primal will cover Barbarian, Druid, Shaman, and Warden
This division is based on there being a total of 16 classes between PHB & PHBII and currently only four power sources. That is unless they introduce an additional power source, such as elemental which could be what Sorcerers use. The additional power sources as listed in the PHB were elemental, ki, primal, psionic, and shadow. If WotC stays with four classes per book then it would make sense that they wouldn’t introduce another power beyond primal so that all the classes balance out between the first four books.
With primal being covered in PHB II that would also leave four powers left to be divided between PHB III and PHB IV. Each source getting four classes would give eight classes per book equaling 16 more classes by the end of 2010 in addition to any campaign setting classes. Due to all the demand and chatter on the boards there’s no doubt psionics will be in PHBIII and most likely either ki or elemental depending on how much noise fans of the Monk have made to WotC.