PSP firmware 4.05 is now available for download. The update will include new music playback visualizations and the ever-conclusive "other features" according to the PlayStation.Blog. According to PSP Fanboy, "Firmware 4.05 includes two new visualizers, one if you have a wallpaper and one if you don't. If you don't have a wallpaper, the waves you see in the background are reduced to one. If you do a have a wallpaper, a water rippling effect can be seen on top of your image. Overall, very underwhelming."
Unfortunately, they didn't discover what the "other features" could possibly be.