I read this some time ago on Reef's XDA thread:
jookdakang23
16th September 2010, 11:24 AM post# 8315
I came across a discovery the other day while playing with android that i dont know if you guys know about or not, but you can use usb tethering and transfer files to your memory card using android!! All you have to do is have the android SDK installed on your computer and make sure ADB works. Once you have the drivers installed on the computers go to usb debug mode on your android, then plug your phone up through usb port and prompt should come up saying mount your drive or whatever! I can provide proof if you need it you can find the SDK here: http://developer.android.com/sdk/index.html
Starting at this post, it appears a couple people had success with this. Has anybody else tried this technique, and had success? Please post with your results.