Skatter Tech has a how to on streaming iTunes to a 2.6 firmware PSP using an iTunesPSP server app - "Sony's PlayStation Portable (PSP) is a very popular device and has sold a few million units, but when compared to Apple's iPod, the PSP lacks storage space. Luckily, the PSP has a built in WiFi card to make up for this disadvantage. With a few simple steps, all of which are included in this tutorial, anyone can stream any non-copyright music from their iTunes library to their PSP." [via] - Link.
Related:
Other PSP projects, hacks and mods - Link.
http://blog.makezine.com/archive/2006/04/stream_itunes_to_a_psp.html