PS3Jig is a homebrew program for the PSP that emulates the official Sony "jig" stick for entering Factory/Service mode. All you need to do is attach a USB cable between your PS3 and PSP, run the program, and turn on your PS3, just like any other downgrade/service mode method.
AS OF THIS WRITING, THERE IS NO WAY TO EXIT FACTORY/SERVICE MODE ON PS3 FIRMWARE VERSION 3.56, SO DO NOT ATTEMPT TO USE THIS ON PS3 FIRMWARE VERSION 3.56 UNTIL YOU READ OTHERWISE ELSEWHERE!
Keep in mind that I'm not a PSP developer, and that I know very little about homebrew for it; my ability to answer questions about how to run it and troubleshoot any issues is severely limited.
This application is open source and has no license attached to it, so you're free to do whatever you want with it; but please don't slap your name on it and call it your own, it's just rude.