Developer cooleyes has recently announced the release of PMPlayer Advance 20080608. This homebrew media player's latest build carries a couple of new fixes. For one, its release for firmware 3.xx has been revised to address issues that were reported with the recently added support for MP4 files. As for the rest of the changelog (translated from Chinese):

* Amended in fw395, the player failed (again mainly nid change)
* Updated version of the fw1xx, playing shots to increase the functions of the player, according to box suspended key players, then circle keys shots, cut out the map png format, stored in ms0: / PICTURE under
* fw3xx version, that some of the compatibility problems MP4AVC

This download is for PSPs running on fw 1.50. The app also includes options for an English interface, so you won't have to pull out whatever translator you use just to get PMPlayer running on your PSP.
Given that there is no readme enclosed with the file, you may want to visit cooleye's sitefor details regarding this release.