PspDirectOnline.info

Dark Alex has released a new tool called the USB Version Fake. This tool is useful for those who download stuff for their PSP from the Playstation Store.

Brief description:
This is a plugin to fake the version that the PSP reports via usb to the PC and PS3. It should let users avoid the firmware version check on the PSN downloader.

By default, the firmware version is set to 4.05, but according to the readme (which you should really check before you install this plugin), you can easily set it to whatever version you need. Installation and usage instructions are also included in the readme.