0okm has released the first 'downgrader' for firmware 1.5 PSPs, allowing them to be flashed back to 1.0 firmware. Catch is, you must currently have a 1.0 firmware PSP to start with, backup the data, upgrade your firmware, mess around or whatnot, and
then you can downgrade back to 1.0; basically, providing you have a copy of your 1.0 firmware dump, you can downgrade back.
Not sure how many people still have 1.0s, but for those who do, rejoice! Hmm, I wonder if it's possible to upgrade all the way up to 2.0, then downgrade to 1.5, then back to 1.0. Woah!
Download FW1.00 Die Hard Trilogy v1.00 from PSP 3D
here.
Check out 0okm's official blog for more information on this release
here.