The real magic of cell lies with its eight "synergistic processor elements," or SPEs, shown in Figure 2. These are specially designed processors created from scratch by the IBM/Sony/Toshiba team just for Cell. They're not compatible with Power or PowerPC code in any way;
It should be noted however that the SPE instruction set and AltiVec while very similar, are not identical. There are different instructions, different data types, the SPEs have more registers and of course the SPEs have a local store.
Just because two chips have similar architectures, it doesn't mean stuff will run without completely rewriting it. At the very least, OS X would need to be recompiled to work with it. While you might be able to compile Darwin to run on the PS3, most of what makes OS X OS X isn't publicly available to do something like that.