It is possible to downgrade a PS3 on firmware over 3.55 but it isn't really an exploit, you have to use hardware flashers like teensy or e3. The process isn't terribly difficult but can brick your device if even the slightest mistake is made.
Not terribly difficult is an understatement @_@ Verifying a correct Dump is a science in itself not to mention that installing the flasher is quite the pain ^^