This program "cleansweep" coded by Canti and dot_blank is supposed to remove the lines from your PSP's background. Readme below.
News found here along with a video.
Im interested in testing it but will admit im a bit on the sketchy side never having personally messed around with the flash before. If anyone does test it, please post your results and maybe a little how-to.Clean Sweep is a tool that will remove the background 3d animation from the psp. As of yet it has only been tested with firmware emulation but will allow for the permenant removal of the bg lines (the tool will allow you to turn them back on) it does however modify the firmware so there are risks involved.