I figured it out, though it sounds like we have different problems. However, just in case this helps you.
First of all your manifest has to have the android:debuggable="true" attribute in the Application node. You probably have this, and I don't know if it actually contributes to the device showing up or not.
In any event, the thing I was doing wrong was when I run or debug my configuration (in Eclipse) under the target tab (where it lists your emulator device setups) I had the Automatic radio button checked. If you flip that to manual it then gives you another dialog when you hit Run, and in THAT dialog, on the top, I see my device.
Lastly, (and I only see this on the Droid X) when you are in PC mode the device doesn't act as though it's connected to the machine. When you're in Mass Storage mode, everything works as expected BUT then the SD card is disabled, so, if your app happens to WRITE to the SD card it will fail at that point in execution. The Captivate (my other test device) does not suffer from the same problem.