PS3 Center has an interesting analysis about why the PlayStation 3 hasn't been hacked yet, you can read it over here. The authors explains Sony has done a really good job on the safety and anti-piracy features of the PlayStation 3, there are basically four big security walls that need to be circumvented: the HDD is encryted and complicated to bypass, there's a 7th Cell processor that double checks everything, there's the encryption of the Blu-ray disc and the PS3's firmware is checked everytime you access the PlayStation Network, if the firmware is faulty you'll get banned from the PSN permanently or worse, your console may stop working. Here's a snip from the article:
If you finally get past the hard drive, you must then face the problems hidden within the actual system itself. We all know the PS3 is a beast with a hearty 7 cells running under the hood as we brag about this on a daily basis. The problem for hackers is how only 6 of these cells are actually accessible, with the 7th cell access being denied to everyone. Not even game developers have access to this 7th cell. Now why is this cell even there if we can't use it? In a simple sentence, the 7th cell runs the PS3 completely on its own. The cell boots the system up, cracks the codes encrypted in all security branches, and finally keeps the OS running while you play a game or do whatever you normally do. Remember how I talked about the PS3 verifying the HDD in relation to the system? This is where that comes into place. The 7th cell is what verifies everything that needs to be unlocked or encrypted. The 7th cell basically double checks that everything in the PS3 actually belongs to the PS3, so users cannot trade hard drives or share illegal games without the cell noticing and denying access. With the exception of communicating with other cells, this cell cannot be written to or acknowledged by an outside source, making it completely secure from attacks.