Are you in developer mode? That is how it will recognize it for quick install, etc.
If you choose USB mode, then you will see the open area of the phone as a USB drive...
What are you trying to do?
Also, be sure you use the Palm branded cable. All usb cables will let you charge, but some cheaper cables have a pin missing (apparently) and it won't see the phone when connecting for quickinstall and the doctor. Blackberry cables seem to work with the pre.
I don't understand why, but I read somewhere on PreCentral that there is more than one implementation of the micro-usb cable and not all of them are pinned the same.