Sony "Un-brick" them using a special NAND-Flash writer via the serial port.

And no, I highly doubt there's a brick prevention system. Sony can't stop the little "noob" from randomy deleting critical system files. If the files are broken, the PSP doesn't boot properly. If it doesn't boot properly, you can't recover it without the use of extra hardware. Also, how do the downgraders work if Sony has a brick preventer? Where do all the bricks come from?

Heh, a pretty silly question if you ask me.