I think that a load of you know what. First of all, it would add a significant cost to already expensive cartridges. Something that could be oggten away with in the 16 bit days when cartridged where kind, and people were used to paying more for games. But not now.
Secondly, it would take a lot more that a new chip to make the SP compare to the promissed PSP. It will take a higher resolution screen, a larger media size, better sound capabilites, and so on. In other words, a brand new machine. Which I have no doubt Nintendo is working on.