TeamXecuter figured out how to downgrade the PSP firmware while working on a project to let you use things other than a mem stick to hold data on for the PSP.
"Using this same development kit we have been able to downgrade v1.52 / v1.51 down to v1.50 or even a v1.00 with no problems whatsoever. We can also tell you its easily possible to patch the kernel to be able to boot anything you like without any type of software loader. It can be acheived in both a modchip AND a non-modchip form so no doubt teams like WAB and others will have solutions for you in the coming weeks as the methods to achieve this are indeed known by others - we are simply confirming its very possible."
"...we made a comment that we had discovered how to do this using discoveries made by dev hardware but we are not going to be making any such hardware to perform this task - there will be no need - groups like WAB will be doing all this via software soon enough."