The difference is that the PS3 will only run encrypted&signed bootcode, which means even if you find a way to replace the firmware, you still need to figure out encryption keys to run your own code. Downgrading might work that way( ie using unaltered signed code from Sony), but forget custom firmwares.how are they doing that with the psp ?
It seems like each time a new firmware is out a new hacked one is out. 3.90 is hacked already , i think it took them less than a week to hack that
On the PSP, if you once got access you can run anything.