WAB is the greatest, not only have they been able to downgrade versions 1.5x to 1.50, they were able to downgrade v. 2.0. As of right now, they do not have a release date for the downgrader due to testing purposes (don't want to brick any PSPs). I have provided the post on WABs website and a link to the video they have supplied of their progress. I will be sure to post when this excellent piece of programming is released and let you know if downgrading my 2.0 was a success or turned it into a paper weight.

It's not the final release of the awesome WAB downgrader, but it's the next best thing - a clear video clip of the 1.5x and 2.0 (Yes 2.0!) PSP firmware downgrader in action from WAB and PSP-Dev. Not only have we found a way to downgrade 1.5x firmware but now 2.0 firmwares also qualify for a downgrade! Incase you are un-aware PSP-Dev and WAB are two European PSP Development teams who will be releasing this superb downgrader, which will allow the running of homebrew code on ALL PSP's.

Source WAB