I'm no coder, just speaking form techno POV. My newly thought downgrader thought theory is on the bottom. If you want to skip the non-sense go to bottom.
From looking at the firmware version. The 1.50/51/52 are somewhat related. The .51 is there just to close the loophole quickly from Sony. And there must be a bug(not neccessary related to homebrew security hole), and there fore the .52 exist. And the assumption that 1.5x firmware upgrade isn't the full upgrade is possible. But generally speaking, most firmware weren't design to be partial upgrade. Assuming Yoshi is still working w/ WAB, the the statement that said "And for those who really want a downgrader TODAY, hey !! just code it ;)" are correct. he must found another hole in the 1.5x firmware. And possibly this hole allow him to messing w/ the existing firmware in the PSP that allow them to re-run the older firmware upgrade(in this case, it would be downgrade) and that is why somebody said it's illegal. Remember, even the 1.50 firmware from Sony is still encrypted, and therefore it should be able to run on the PSP. Also, because of that, running homebrew cannot be use with this exploit.
Running a homebrew using gameshare method is the exploit in the DS. Not saying it isn't exist in the PSP, but unlikely. Also, 1.5x F/W should still stop the execution of the unsign regardless of where it originate from. If not, it probably already been done. If I am the Sony firmware writer(fixer), I wouldn't make a check routine at reading from the media, but right before executing the file. Google the "passme" and "wifime" if you don't know what I'm talking about. Even if it's possible and haven't been try. Then again, the need for a PSP w/ 1.0 or 1.5 doesn't neccessary exist. Someone could make a PC software that simulate the PSP data stream and send it over to the 1.5x PSP. And bingo, same effect as the gameshare method, except no need for the second PSP.
On another though, the 2.0 firmware must be rewritten from scratch, and therefore it's a full upgrade, all the neccessary files must be there and avialable. Of course, it would be illegal to hack that firmware(not that it will stop anyone for trying). The decoding/recoding the 2.0 is the easy part(for many brilliant coder out there.) The hard part is decrypt/re-encrypt(sign) the firmware again.
My 3rd possible exploit theory.(just a thought, and probably be illegal also)
is similar to the gameshare method. except it's a man-in-the-middle attack.
I think the PSP can connect to the Sony server and grab the firmware to the MS b4 running it. What if, you reroute the data, instead of going to the Sony site and grab 2.0 firmware, to go grab your choice of firmware on your harddrive. And because the firmware is loaded from Sony site, there MAY NOT be a precedure call to check for the downloaded firmware.
Again, I'm no coder, and don't want to risk brisking my PSP(even though it's still at 1.51), so I'm not even gonna try it. But all brave coders out there(who have many PSP at their disposal) are welcome to try my method.