This is something that interests me too. It sounds like there may be a way to get it into USB Host Mode to use a USB keyboard, but being able to mount a flash drive sounds like it would take some kernel level drivers. If the Dell Streak actually supports mounting a USB drive, that would be awesome.
I have a handheld DeLorme PN-40 GPS receiver that can be seen as a drive on a PC. It would be awesome to be able to connect it to my X. I'd love to be able to download GPX files to my X and transfer them to the PN-40.