If you have upgraded your Sony PSP to firmware 2.0, you apparently can now downgrade again with a newly surfaced Sony PSP hack.
There are of course many more homebrew programs running still on Firmware 1.5. This is the only reason for some users to downgrade.
Such things are risky to do (Sony will not help you if your PSP gets screwed up), but the word is that it is supposed to work.Download and instructions for the firmware downgrader are available on PSP Updates.
See also the recent story on the first Sony PSP Firmware 2.0 exploits to run homebrew code.
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