or use pspvideo 9 it also renames files, or us a a program that renames loads of files into the type that the psp needs (for instance: http://www.mediachance.com/free/renamer.htm), which is:

M4V1000number.mp4 ("number is hopefully self-explanatory") into
\MP_ROOT\100MNV01

Again if u just use pspvideo9 there's no problem.