Ok, I have wrote about this before, but had little time to write and nobody was able to understand my theory.
So, here is a rewritten one that hopefully makes more sense.
Right now it is possible to use InFeCtuS to downgrade a PS3.
This is done by copying the NAND chips to a file so you can write the file back to the NAND chips when you want to go back to your previous firmware.
The PSP was hacked so someone could take advantage of the tiff exploit and swap index.dat from flash0 to the index.dat from the 1.00 firmware, tricking the PSP into launching the 1.50 update EBOOT. (index.dat tells the PSP it's firmware # so if replaced with the index.dat from the 1.0 firmware the PSP thinks it has firmware 1.0)
But, what if someone made a program that could change individual files on the NAND chips?
Could this be done on a PS3? If you could replace a file that gives the PS3 it's firmware number with an older one wouldn't the PS3 launch the Sony updater that is older than the PS3's current firmware?