x86 and derivatives will never be replaced. The chance of it happening is the same as Windows being replaced by an alternative.
Intel tried to replace x86 with Itanium (and even had Microsoft's support for it)... and failed because AMD extended x86 to 64-bit.
The ISA used in the Cell (POWER-based) has been around for a long time and Microsoft hasn't shown any sign of switching - if Microsoft doesn't endorse your CPU architecture then you have zero chance of entering the desktop, as long as Windows is the dominant platform.
If we had an open OS like Linux, BSD, OpenSolaris, etc. then a PC manufacturer could sell PCs with a different CPU ISA and all of the applications ported to that ISA because their source code is open. Again, since a closed-source OS is dominant...
As for the Cell as an add-on chip, it will be replaced by GPGPU (CTM, OpenCL, CUDA, Larrabee etc.).







