The wall he was referring to, is related to the heat generated by the processors.
Because an Intel processor, say the pentium 4, has to run a lot faster than a powerpc processor to calculate the same amount of data, they also all run a lot hotter.
The are now running so fast, that predictions say it won't be long until the cooling systems needed to enable them to run will become to costly to be practible.
Powerpc's on the other hand crunch a similar amount of data at lower speeds (I think because they compute a larger amount of data every clock-cycle)...
So Jobs is basically saying: "our way is better..".