The problem isn't so much that it's hard to develop for ps3, but rather that you have to develop on the ps3 in a completely different manner than you would on the 360 to maximize the performance of the system.
Porting games from 360 to ps3 or porting games from PC to either won't really work. The ps3 is at it's finest when developers heavily use the SPUs to assist the RSX with graphics functions, and when they use heavy streaming from the Blu-Ray disc. This is why games like Killzone 2 and Uncharted are some of the best looking and smoothest running games out there (Uncharted has no loading whatsoever).
Call of Duty 4 also performs excellently because Infinity Ward had two teams dedicated to creating an engine for both platforms, rather than creating the engine primarily on PC, then porting it to each system.
Porting a 360/PC optimized engine to ps3 just won't work, because developers will use the HDD as a crutch instead of significantly tailoring the engine to the ps3's hardware. It's also one reason why many multiplats look worse on ps3.
