I came across an interesting paper called Deferred Pixel Shading on the Playstation 3, by Alan Heirich and Louis Bavoil. They used the RSX as a pure rasterizer to build the G-buffers, then ran a pretty complex shadowing algorithm on five SPUs. They achieved 30 giga-ops (note that they don’t quote GFlops, which are much more commonly used to measure performance in this field - this is surely intentional) and around 11 GBytes/sec data transferred around the system.
Let’s convert this to more familiar terms, pretending that their “ops” are actually “flops” (there shouldn’t be much difference anyway, from what I know about the SPU instruction set). A game running at 30 fps in a 1280×720 resolution, without antialiasing, needs to shade 27.6 MPixels/sec. If you use 5 SPUs, like the authors of the paper, and achieve the same throughtput, this means you’d have about 1000 operations per pixel; given that traditional GPU pixel shader instructions are usually four-wide, this would be roughly equivalent to a 200-250 instruction pixel shader. On the bandwidth side, you would have about 400 bytes per pixel. If you use, say, four 32-bit surfaces for your G-buffers - which is what I remember as normal from the deferred shading papers I’ve read - and want to write another 32 bits to the final framebuffer, this leaves you with over 300 bytes of extra data to shuffle around - various shadowbuffers, several passes etc. 250 instructions for the lighting shader itself is also pretty generous, even though it would have to be divided among several passes. (You’d realistically want to do MSAA or even SSAA for a real game, which would raise the bandwidth and computational cost significantly - but on the other hand, neither the 30 Gops nor the 11 GB/s are anywhere near the theoretical throughput of Cell.)
All in all, I fully expect to see games doing deferred shading on the Cell before the generation is over. You “just” need to come up with a renderer, scene, world and game design which can utilize the strengths of the deferred shading fully - so the title would stand apart from the forward-rendering crowd, which would justify the pain of getting this to work. But on paper (pun intended), the numbers add up - it definitely seems possible.