False. The difference is that only Sony developers tell people outright their SPE useage as some kind of bragging platform. X% use of SPEs have at you! Its obvious that multiplatform developers have been making use of the systems on both systems but they are just a little behind exclusive developers. Edit: Based off the progression of multiplatform titles over time and comments made by multiplatform developers.

1. I somewhat doubt multiplatform developers are willing to jump through hoops to get some rather unique (to say the least) results out of the Cell when the same could not be easily replicated on other platforms.  Hell, that's the reason so many PC devs aren't supporting the ps3 at all (Valve, Piranha Bytes, Gas Powered Games).  They just can't be bothered to do things in a new or different way.

2. If the engines for the three platforms began diverging that much (having functions being run on the Cell being completely rewritten to run on the Xenos/Radeon/GeForce), it wouldn't be a multiplatform engine so much as one game being built on two engines (ps3 vs pc/360) that happen to share a similar basic structure.  Though I suppose you could still consider it one single engine - a very versatile engine, to say the least.

3. The only time I've really seen a multiplat developer brag about their Cell usage were Pandemic after completing the Saboteur, and the only difference between the two versions of the game that I know of was the implementation of AA.  Meanwhile, first parties are getting AA, lighting, and various post-processing effects all running on the Cell in their games.

4.Getting the CPU to fill in for the GPU is simply something that is not easily replicated on other platforms, and its not economical to do things an entirely different way on one platform vs another.  Instead of doing a port you're building a game from the ground up for both systems, and this will lead to major differences between each version of the game (and thus an inevitable backlash from one or both camps), while also adding to the cost of developing the game.

It just doesn't make sense for devs to go down that road, though more power to them if the try.

http://repi.blogspot.com/2009/11/parallel-futures-of-game-engine.html#links

This should pretty much answer 1,2,3.

I'll add: 1. Its due to the fact that they don't have to. Valve are supporting a legacy environment from shader model 2 GPUs up to shader model 5. Its simply a lot of work to make sure the games run well enough on old single core PCs before adding more models in. The timing of the PS3 comments are probably relating to them embracing more a multicore approach as their userbase finally migrates.

2. If you recompile an Xbox 360 game into a PS3 game and vice versa, barring bugs etc it will run. We're not talking 30%->50% of the code here, we're talking the difference between executing post processing on the Cell vs on the Xenos for the Xbox 360 for example. See the presentation for details about their 1.5M lines of code duplicated between PS3/Xbox 360/PC.

3. See presentation for Cell useage, I believe its higher than the Killzone 2 useage so does that not cast down the "will not run on Xbox 360" down to the sodomites where it belongs? The multiplatform developers are getting much higher utilization out of the Xbox 360s GPU so they are running said post processing on the GPU whilst the shaders are idle because they do not involve significant/any texture ops.

4. Its been typical practice since day 1 on the PC. Have you heard of JIT compiling for the PC platform? So long as each platform has an efficient way of running the code or slow running processes can be made up elsewhere in the chain theres no reason why they can't use the same effects. They are both computer systems after all and theoretically the Xbox 360 and PS3 can do everything each other can do albeit at varying levels of competency.