My solution
I wish phungy had replied as well, because I just had this problem. I'll post my solution.
My USB drive failed to be recognized after OS X said I needed to force eject it because something was tying up the unmount process. I'm not sure what that was, but force eject wouldn't work either and I had to physically pull the USB drive out. After that it wouldn't be recognized.
My solution was Disk Utility, which did recognize the drive and said it was unmounted but "couldn't eject because it couldn't unmount the disk." After a couple tries with the eject (button on top of Disk Utility) the drive suddenly showed on my desktop in a grayed-out state (in the process of being ejected) and then it disappeared from Disk Utility. After that it worked normally.
I can't quite tell what happened but fiddling with the eject function in Disk Utility did the trick.
Jeremy