The iPhone is thought to have 3 to 5 processor cores inside it. Touch, maybe 2 or 3. More than one CPU in the iPhone is an ARM core according to an EETimes report. The iPhone a small multi-processor system, and the OS install package includes kernel code for more than one of those processors. The application CPU in both the 1G and 3G runs at 4xx MHz and has 128 MB of RAM. Nobody (except Apple and the chip designers/vendors) knows how fast and how much memory the other CPU cores have in the 1G and 3G, or how different they might be.
If those other CPUs aren't the same, that's a strong technical reason why 4.0 support can't be ported to the 1st gen devices.
I'm 99% certain that the iphone, and iPod touch... all models have ONE processor, and ONE core. Not 3-5