Well, as most every psp site is reporting, it’s (un)official and I can confirm it works : using a libtiff exploit (meaning that a psp picture library program that deals with how to display ‘tiff’ image files is not dealing correctly with certain types of tiff files) you can now downgrade your Sony psp from firmware 2.71 to 1.50.
Please note that you HAVE to have firmware 2.71 – I tried literally dozens of times with 2.70 and it did not work. Once I upgraded to 2.71 however (you can still find it on the net if you look hard enough) the downgrade happened after about the third time I tried.
If you then install DevHook again (on your 1.50 firmware) you gain (again) the ability to play the latest games, as DevHook will run firmware 2.71 from a dump on your memory stick !
Cool ! I am gonna explore Homebrew again a bit, seems like there has been a lot of advancement since I last checked, almost 8 months ago or so.