Two totally different architectures. We're not even talking converting PowerPC instructions to X86 instructions here; we're also talking about emulating the 6 SPE's in software, which is near impossible due to how they access memory (via DMA).
Most platforms need 10x their power to be emulated effectively, but because of the PS3's very unique architecture, it will probably need twice that! So maybe in 20 years or so we'll have a decent PS3 emulator out.