PandoraThat PSP homebrew/hacking scene is pretty amazing. This new thing can downgrade any current PSP hardware to 1.50 for homebrew use, including PSPs that have previously been bricked! They apparently reverse engineered the Sony service mode using special software and...a battery hack? Wild.
Now while it will work for any current PSP no matter the firmware version on it (including future firmware releases), they aren't sure if it'll work on the new Slim PSP models. Depends on what hardware changes were made, I suppose.
The downsides are first, the battery. Once converted to service mode it can't be changed back and used as normal. So you need a separate battery for this task. The other problem is you can only create the battery on a PSP already capable of homebrew.