No, this isn't a dream. As of today, there is a working 2.50/2.60 firmware downgrader!
Based upon the initial work by Dark_AleX (which caused quite a buzz due to its dangerous bricking tendencies), the downgrader is also the work of Yoshihiro and Mathieulh of SonyXTeam.
Mathieulh announced the downgrader yesterday, and it has been tested by dozens of people on
Fanjita's IRC channel, including ourselves.
The downgrader requires that you acquire a dumped version of the 1.50 firmware. Unfortunately, these files are copyrighted; you'll have to get them through less-than-legal means, or from your own 1.50 PSP. If you have a 1.50 and want to make a dump for yourself, grab
the 1.50 helper program. Make sure you have about 15MB of space on your memory stick, then run it. It'll create a folder on the top folder of the memory stick called DOWNDATER, containing a full firmware dump.
To use the downgrader, follow our
guide. Please note that you will need an additional 20MB or so of space on your memory stick to attempt this, as the downgrader generates several large log files.
Do NOT attempt to downgrade with a 32MB memory stick!
Props to all who helped make this development possible. Teamwork pays off!
Download the 2.50/2.60 Downdater [Downgrader] FINAL Auto Version from PSP 3D
here.
Download the 1.5 Firmware 2.50/2.60 Downgrade Helper from PSP 3D
here.
Password for the downgrader archive is: I_Use_It_At_My_Own_Risk
Note: As more reports come in, this article will likely be updated to reflect them. UPDATE #1: It is apparently safe to try this on any PSP that came installed with 2.50 or 2.60,
so long as it is not a TA-082 hardware version. UPDATE #2: We've had a few confirmed bricks. A user named 'Stoicism' on Fanjita's IRC channel stated that his memory stick light flashed for about 15 minutes, and wouldn't stop. Upon a hard reboot, it simply flashed the memory stick light at evenly-spaced intervals. IRC user 'Meddle' says his PSP refuses to load after running the downgrader. Possibly not TA-082 hardware?
UPDATE #3: A video of the entire process has been released on YouTube.
UPDATE #4: The FINAL version of the 2.50/2.60 downdater (downgrader) has been released, featuring a few additional checks during installation and safety precautions.