Patty O'Furniture claims to have successfully downgraded a PSP.
NOTE: PSP-Spot does not endorse this method nor take any responsibility for any potential damage to your PSP. Use at your own risk.
"Ok here's what I did, I have a 1.51 PSP from Europe and it comes with a UMD that has the 2.0 firmware on it. So I put in the disc and started the upgrade, and at the point where it said "make sure the PSP is pluged into an AC adaptor" or something like that I can't remember it word for word. It is the last point at which you can cancel the upgrade. Anyway before I continued I removed the disc and inserted the Midnight Club 3 disc and then continued. Being that it had already bypassed the version check it continued to load the firmware thinking that it was the 2.0. Now I know that this works because I tried it on three of my friends PSPs all 1.51 I tried it on mine first so I wouldn't brick theirs. And all of us can run homebrew cool! I am guessing that this will only work on 1.5x versions seeing as how you can't use the 2.0 disc to upgrade one with 2.0 already on it. I hope someone will find this helpfull."
this is from PSP-Spot
Does anyone know of any UMDs that have 2.00 or 1.52 update on them?