Dark_AleX has released an update to the fatmsmod patch today. Apparently it stopped working in 5.00 due to a Sony file changing its name.
Download: PSP FATMS Patch for CFW 5.00 M33 / PSP FATMS Patch for CFW 5.00 M33 [Mirror]
From the ReadMe File:
Instructions: decrypt 3.71 using psardumper (square option).
Copy the file fatmsmod.prx in the root of memory stick.
Copy FATMS371 to /PSP/GAME or /PSP/GAME5XX.
Run the program and done. More PlayStation 3 News...