A USB microphone would be very cool, indeed.
I've seen lots of microphones that plug into an iPod's headphone jack, like this one:
http://www.amazon.com/VC-00300-VeriCorder-Mini-Mic/dp/B001Q964QI/
I checked with Vericorder, though, and they said this:
"No, the Motorola Droid headphone jack...