UPDATE: Since this how-to was first written things have changed. Sony has released firmware 2.01 which makes this downgrade impossible. A PSP bricker trojan was released because of this antivirus software will classify both the good and bad downgrade files as harmful. You should do more research into the current state of the downgrade before attempting. As far as we know our firmware links are still good, we are not hosting them so we can not vouch for their safety.
C.K. Sample, III, TUAW / WIN Blogger and author of the forthcoming book, PSP Hacks, got his hands on the version 2.0 downgrader and wrote up a thorough how- to complete with pics for us. He even takes the extra steps to upgrade and downgrade yet again, just to make sure such a thing is possible.