detailed (translation) by an unnamed French developer working with the beta version of the firmware. 5.00 will supposedly include direct access to the PlayStation Store, which was promised last month for the fall (in Japan, at least), along with an improved RSS reader with text file support and a screen capture mode that supports in-game screenshots. The source could not confirm a date for the firmware's public release, but speculates that it could coincide with the launch of PSP Brite, the new hardware revision, on October 14th or in the weeks following. PSP's current firmware version is 4.05 -- if true, it would be a big leap to 5.00, by Sony standards.
[Via PSP World; thanks, Skylar]