Sony didn't want us going back to firmware 3.60, Freeplays made my own installer. Using an encrypted, un-sigchecked dump of firmware 3.60, the 3.60 IPL, and a few modules from the 3.52 updater, you can now install 3.60 again on slim PSPs.
Read more about PSP Slim Firmware v3.60 Installer v2 (PSP Application)