Confirmed v0.5 works on PSP1000 which came with 1.50 upgraded to 2.60 then back to 1.50.
v0.5 loads black screen, MS light flashes for about 3 mins, psp automatically reboots, come on stating memory currupt, then reboots again to welcome you with the psp setup screens just like a clean psp from the shops.
Just search for GTA EBOOT Loader on [Register or Login to view links] and you'll find several... then read the descriptions to decide which is best for you, for reference v0.9.7 is one of the more recent ones. Please read the ReadMes and figure out how to use them on your own though- I DON'T want this thread turned into a "How Do I Use It..." so keep this topic on Post Your Results: Dark_AleX's 2.6 Downgrader Test guys.
i tried the downgrader and all went awesome, memory stick light was supposed to go out and then restart my psp, but it never did go out it stopped for 2 seconds and then this screen popped up in some weird language all i understood was to press "o" so i did and changed nickname and language and checked system information and wala 2.5 PSP downgraded to 1.5
totally works take me hat off to you lot, first attempt broke my black psp 2.6 1001, worked on my 1000 white 2.01 version upgraded 2.6, it took me balls of steel to try again but its worth it i have a 1.5.