Short Answer: The USA-SPEC PA11HON2 (
http://www.crutchfield.com/S-obnbr4N...c&i=581PA11HON )absolutely works in the Base Fit and it's not too tough of an install. It plugs into the back of the head-unit into the CD changer port (14-pin) and is accessed by hitting the Aux button on the radio. You can control the iPod functionality directly or through the head unit - your choice. It also has RCA inputs for extras or for future use with a next-gen music player. I'm not sure how this unit would interact with the OEM Aux line-in on the Sport... this is something to test! Note that the item you posted from Circuit City is for Panasonic head units and not for OEM Honda Fit stereos (ie. it won't work).
Long Answer: Full writeup on PA11HON2 install in the Base Fit coming soon (with pics)
Hope this helps!