The processor and the ram are stacked on the same chip. So they're kinda limited as to what they can put in there. I doubt that you could fit a SO-DIMM of any stripe inside an ipad case, nor would you want the power problems.

There was a teardown of the chip assembly done back when the ipad came out that has some good details: http://www.ifixit.com/Teardown/Apple-A4-Teardown/2204/1