At this time there are no released soft downgrade method for newer firmwares, but I do believe there is one for 3.56. However you can hard downgrade any firmware with a flasher as long as it is not below the firmware your PS3 came with.
Thanks I've been wanting to downgrade for ages now... but I don't have much money... :/ Is dual booting possible for the E3 flasher yet? (without soldering)...