There's a book about the development of both of them, The Race for a New Game Machine: Creating the Chips Inside the XBox 360 and the Playstation 3 by
David Shippy & Mickie Phipps. [0]
It has some details on the awkward position the IBM developers were put in.
Rodrigo Copetti's excellent articles on the PS3[1] and Xbox 360 [2]
and Ars' Hannibal's article on the Xenon Chip. [3]