PHB3 Revealed
BrentB 19 May 2009 21:46
Apparently full, final, playable content will be released through D&DI starting in July and ending in March 2010 when PHB3 is released. A bit of a spoiler is the cover of the book. It says "Psionic, Divine, and Primal Heroes." Also on the front are a githyanki and a minotaur, presumably new playable races.
To my mind that pretty much cements the five power sources. A sixth introduced in a PHB4 seems very unlikely.