Well, here's a new reason. Dr. Dobb's Portal, a trusted computer research site, published a report that shows that the Cell processor (the heart of the PS3) is up to 22 times faster than the leading Intel super-computer chip, but that since it processes information in a different way, it can be more difficult to program for.

While the Dr.Dobb's report is over a month old, GamePro is providing new quotes from Sony that shed light on the subject. According to Sony officials:

There is a trade-off between performance and versatility, since PS3's Cell processor allows more features -- better physics, more complex graphical processing, lighting or sound -- there is inevitably going to be more cost in supporting those extra features. It's not that PS3 is harder to write for, it's just that you can do more with it. If a game starts life on PS3, then man-hours per feature or costs related to asset production are comparable with industry norms.

If your game starts on Xbox 360 you will have to re-engineer aspects of the game to run properly on PS3. This means additional effort. Some developers have been complaining about this, but I don't believe we can solve that. The 360 is a different machine with good, but lower powered hardware in a different architecture. Developers have to view them as two different machines not as a common platform.