Would it be possible for a programmer to make a USB host driver for PSP? It could be realy usefull! Play with the thought you plug in a USB-chassi with a 2,5"hdd and launch the custom made filemanager and transfer files to your MS. Perfect when you travel!
You cannot make the USB port act as a host in software unless the hardware supports it, which from what I hear the PSP doesn't. The USB keyboard for PSP that has been demoed actually worked as a USB host.
However, there are several portable HD products available capable of acting as a USB host or even copy directly to the memory stick, like the http://www.apacer.com/apacer_english/product_html/share_steno_cd311.asp and others.