If you're not content in going from 2.6/2.5 to 1.5, a 1.5 to 1.0 downgrader is set to be released tomorrow. This original firmware that was released on the Japanese PSP at launch allows you to run unsigned eboot files sans exploits. Check here tomorrow to see if everything goes according to plan.

Check out the video to see a live 1.5 -> 1.0 downgrade.

1.50 -> 1.0 Downgrader in Progress [PSP Updates]