The Cell processor has an SPI port which is used to configure the chip on startup. Well documented here. It also allows hypervisor level MMIO registers to be accessed. In the PS3, the south bridge sets up the cell, and the traces connecting them are on the bottom layer of the board. Cut them and stick an FPGA between.
Quick theoretical attack. Set an SPU's user memory region to overlap with the current HTAB. Change the HTAB to allow read/write to the hypervisor! If that works it's full compromise of the PPU.
A Real Challenge
The PS3 has been on the market for over three years now, and it is yet to be hacked. It's time for that to change.
I spent three weeks in Boston working software only, but now I'm home and have hardware. My end goal is to enable unsigned code execution, making every unit into a test and opening up a third party development community, either through software or hardware (with a mod chip). The PS3 is a prime example of how security should be done, very open docs wise, and the thing even runs Linux. But it isn't unbreakable :-)