There were IBM, Sony, and Toshiba engineers working on the initial PowerPC core. That was designed for the Cell. But when Microsoft came in and said, "We want something similar, but with all these extra enhancements," we convinced the Sony and Toshiba partners that we wanted to do this one, common core that had these new features and they agreed to that. -Dave Shippy
This quote comes from the fourth question of the interview. He later mentions that further modifications requested from Sony were added. IBM's goal was to use a common core, albeit within different CPU designs.
Naturally, he's approached with fanboy extraordinaire question, "Which [CPU] was the more effective or powerful?" Intelligently, he responds that both processors are "fairly equal".
The last question involves an MS vs Sony business approach comparison. Both companies shot for 2005, and IBM met their goal. MS's was very aggressive and had backup plants (one of which involved relying on software where hardware wouldn't suffice). Sony was conservative, which left to many delays.
For more info, pick up the latest Game Informer and/or get Dave Shippy's book, "The Race for a new Game Machine".
[Dave Shippy interview, page 38, Game Informer Issue 191]