Chilly Willy updated his PSP Doom port:

Changes:

v1.3 is up! Networking finally works; you can also play against compatible PC/Mac/Amiga versions of DOOM. The GUI has been revamped to make it easier to set up and play a network game. You can now set the starting level and skill.

http://groups.google.com/group/chilly-willys-ice-flow/web/doom-for-the-psp?hl=en