The same problem applies to any regular USB stick or hard drive, the hardware is simply incapable of doing the job.
Porting it to the PSP is a different problem altogether - not only do we not have any documentation on the USB Controller for the PSP, and the chipset has limitations that most other devices do not. It'll get done *eventually* but it will take a lot of hard work AND luck.. the devs are mostly fumbling in the dark trying to code a PSP-PS3 jailbreak.
There will never be a "normal" USB version. It may someday be possible to do this from a laptop or netbook running Linux, but in order to do so you would have to render all the USB ports on it inoperable by turning the USB controller into a slave. This is all assuming we can find a laptop/netbook that even has hardware compatible with making the USB ports into slaves. I'd think that would be a low priority when people are designing a laptop, given that you want the ports to actually