I've looked in the direction of doing that myself as well, but there are some ugly problems, as the connection is not exported as a "drive" per se, but as an Explorer Namespace Extension. You will find that your PDA is visible in the Explorer not as a drive (letter) but as something separate, and that (in most cases) the behaviour when copying files is different from the standard behaviour, as it is programmed by the implementors of the PDA driver.

The direction you need to go is to create a (pure) COM interface to IShellFolder and/or IShellBrowser, which will mean some C or C++ wrapping and much reading of the MSDN documentation. If you are lucky, you can grab whatever communication your PDA tools do, by installing a wrapping USB sniffer, or by looking at the locally cached files.