I got mine working lastnight... just want to double check, after you connected the Droid via the USB cable, did you go to the Droid screen pull down the notification panel and select the USB notification? This stumped me at first because I am used to things just showing up but it won't show up until you click on the notification. Hope that helps.
1) Connect the Droid to your computer via the USB cable. A USB icon will appear in the upper left corner of your phone.
2) Drag down the notification bar and click the "USB connected" entry. Now click Mount. Now the drive will show up. On a Mac it will be on your Desktop. On a Windows computer, it will be inside My Computer.
3) Once you are done transferring files, you need to eject the disk from your computer. On a Mac, right click it on your Desktop and choose "Eject". On a Windows computer, click the green arrow icon in the lower right near the close and safely remove the drive.
4) Drag your notification bar on the Droid down again and click "Turn off USB storage" and then click "Turn Off"