The Streetpilot III is part of the older generation of Garmin GPS's and just happens to be perfect for what you want! If you look on the backside, there is a round, four conductor plug that had Ground, Power, TX Data and RX Data. This port is true RS232 serial data, perfect for APRS. The SP3 is capable of both NMEA and Garmin Binary data.
The first most "appliance user" way to do what you want is to buy a Kenwood D700. This radio is APRS ready and only needs the additon of the GPS. A simple cable that interfaces the GPS and radio for bi-directional serial comms serves to supply the radio with position data as well as sending $GPWPL strings back to the GPS for mapping. Basically, when the radio decodes a packet, if the packet has position data, the D700 will take that data and format a $GPWPL waypoint string using the callsign as the waypoint's name. My company RPC Electronics (www.rpc-electronics.com
) manufactures the cable needed to inteface the Garmin to a D700.
You're next option is a soon to be released device from Scott Miller N1VG (www.n1vg.net
). Its the OpenTracker II that is a full TX and RX TNC with waypoint support. I like this option because you can use the radio you want. Scott even support native Garmin Binary which will actually use an icon close to the object's instead of a plain old block, like the D700 uses.