First thing that pops into my mind would be altering the game files. Changing the damage and fire rate on weapons, etc.
I have no problem with Sony banning modified consoles from PSN. What I have a problem with is their strict attitude towards their (legitimate) users. They keep taking things away like OtherOS (I literally chose the last fat ps3 in stock over a slim for it), 3rd party controllers and devices (I need to check is my ps2 controller converter is working still), and their intrusiveness into your gaming device (sending data about all devices connected to your ps3 - yes they want and do collect info about your shiny new Sony Bravia TV and what accessories you've purchased).
I mean for all intents and purposes the ps3 has been "rooted". Wouldn't the ideal way to go about this be to patch up what you can (PSN/Multiplayer) and try to win people back by doing something like bringing OtherOS back and ADDing features? They will never be able to "fix" this so shouldn't they try to add value to not using hacked firmware?