(This screeshot is of the DownHelper when run on a PSP v1.5)

Update 5: Looks like v0.4 produced another brick, so its back to the drawing boards.  Also, 0okm, the dev responsible for the 1.5 to 1.0 downgrader has apparently been able to downgrade 8 of 10 PSPs sucessfully down to v1.0 from v2.5/2.6.  Here are the only quotes of his progress, from the latest quote to the oldest:

"sorry
it is unstable
i test 10pcs FW2.60 PSP
8pcs OK
2pcs have error

i can\’t share it
i don\’t want to have Dark_AleX\’s condition "

"yes
i can confirm can use hitchhikr\’s concept to "downgrader" old ver. hardware PSP with FW2.60 to FW1.00
i was use another method NOT Dark_AleX\’s Downgrader"

"if "ookm" is "0okm"
i can tell you
i try my way with hitchhikr\’s Great Work ^o^

"i test 10pcs FW2.60 psp
reflash them to FW1.00
8pcs OK
2pcs have error
but never mind
i have multi FW Module ^o^"

Update 3:  Looks like DarK_AleX and some other devs have been hard at work debugging the new downgrader, and now they have hit version v0.4!  There is a willing tester ready to try it out, so I will keep you updated and post the video as soon as its available.  In the meantime, check out the changelog for all the versions:

Yep, you heard me, DarK_AleX, the same person behind the NethostFS PRX, USBhostFS ISO playing, Firmware v2.6 Decryption, and of course the infamous DAX zISO Loader has done it again!  While Fanjita has been feebly trying to get his eLoader to be able to operate with kernel memory access, DarK_AleX has blown him out of the water by releasing a downgrader for PSP users with Firmware v2.5 and v2.6!  Currently considered TEST, it is in a state I would call use-at-your-own-risk-with-a-non-sony-extended-warantee.  There have been two successful attempts reported (though with some unknown quirks), and two unsuccesfull ones which were probably the result of negligence in following the rule of making sure the PSP does not have a TA-082 motherboard.  Anyways, I encourage our brave members who have nice solid extended warantees to try this out and report back in our forums with a thorough report, it would be awesome if someone could make a video of their entire attempted downgrade.  Once again however, I will remind you that neither DarK_AleX or PSPBrew assumes any responsibility for a bricking, so make sure you carefully read the entirety of the following readme and follow it to the "t".

Downgrader Test For PSP 2.50/2.60 Firmware
by Dark_AleX

Download link:
http://rapidshare.de/files/24404395/downdater_test.rar.html

Password: I_Use_This_at_My_Own_Risk

To be used at your own risk. It is an UNTESTED software!
This is an *experimental* downgrader for 2.50/2.60. Experimental means here that it has not been tested on a real device, althought it has a theoretical base that it could work.  This software is free and untested, and i\’m not responsable if it causes damages to your PSP. If you use this software, you agree that you are using this program at your own risk and that you won\’t blame the author for that.

"Someone told me that a downgrader for 2.50/2.60 was impossible. How does this work?"
This one uses another more risked technique (and i repeat: untested) than the MPH downgrader. It uses some sony libraries found in the 1.50 updater to perform the most critical steps, the logical format of the flash, and the ipl writing. See technical details for more info about how it works.
This software requires that someone with a 1.50 runs a little program and send you the output so you can init the downdate process. The 1.50 program is TOTALLY safe, it only dumps the flash of a 1.50 user and it also extracts some special prx\’s found in the 1.50 updater.
If you do not wish to have someone with a 1.50 do it for you, you can use the included MS_ROOT folder with the files in it instead.  Inside is the "DOWNDATER" folder you can use with a 2.50/2.60 PSP to test the downgrade.

Instruction for the 1.50 user
—————————–
1) Copy the folders downhelper and downhelper% from the folder 1.50 HELPER folder to
/PSP/GAME/ in your memstick.
2) Obtain the 1.50 update EBOOT and put it in /PSP/GAME/UPDATE in your memstick.

Download link:

http://www.psphacks.net/files/PSPFirmwares/1.5/
http://dl.qj.net/PSP-Firmware-1.50-File-(EBOOT)-North-America-PSP/pg/12/fid/163/catid/163

3) Init the downhelper program.
It will dump your flash and some files from the updater eboot to the memstick.
4) After that, the program will exit. You can now delete the updater from /PSP/GAME/UPDATE
5) You\’ll notice that you have a new folder in the root of your memstick called "DOWNDATER".
That\’s the folder you\’ll have to send to a 2.50/2.60 to let him test the downgrade.

Instructions for the 2.50/2.60 user
———————————–
1) Copy the folder the DOWNGRADER/2.50/DOWNDATERTEST if you have 2.50
or DOWNGRADER/2.60/DOWNDATERTEST if you have 2.60 to /PSP/GAME/ in your memstick
2) Wait someone with 1.50 to pass you a folder called DOWNDATER and copy it to the root of your memstick.
3) Init GTA and the GTA Loader
4) In the GTA Loader menu, choose the downdater test. WARNING: the program won\’t output any display
and any warnings, it will init the downgrader process inmediatelly.
5) You\’ll see the your memstick flashing. That means that your PSP is being flashed from the memstick.
You won\’t see any type of output in the screen (this is for safety). When the memstick finishes of
flashing, WAIT at least a minute, and then reinit your PSP by holding the power button.
6) That\’s all. If all went right, you\’ll have 1.50. If it went less good, you\’ll still have
 2.50/2.60. If all went wrong, you\’ll have a broken PSP (except in the case you have
the mod-chip) Remember that you have accepted that risk.

Technical details
——————
This program performs the following operations.
- It loads the ipl_update.prx and lflash_fatfmt.prx from the 1.50 updater.
- It performs a logical format of the flash0 partition using the function
  sceLflashFatfmtStartFatfmt found in the lflash_fatfmt.prx
- It writes a dump of the 1.50 flash0 to the flash0.
- It writes the Initial Program Loader (Ipl) using the functions sceIplUpdateClearIpl
  and sceIplUpdateSetIpl from the ipl_update.prx module. The ipl is embedded in that module.
Other considerations:
——————–
- Users with TA-082 psp\’s shouldn try this. Also, it may be better that at the beginning, only users
with a 2.50/2.60 NOT from factory (they updated from a below firmware) try this.