From what I have read, there is not yet an RS-232 serial cable available that will fit the 3800 series of IPAQs. The page does not mention that they are compatible with this cable (236251-B21). I've been tempted to get a 3850, but I'm going to hang on until I know for sure that I'll be able to hook my eTrex to it.

http://www.compaq.com/products/handhelds/pocketpc/options/pc_connectivity.html