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Last night I finally got around to making a computer interface to let my laptop, with the help of soundmodem, do packet radio with my FT-897.  This simple circuit (mostly stolen from here) lets me do just that.  I connect this to the DATA port not the CAT port on the back of the radio, allowing me to still use the CAT port to control the radio itself.  My additions to this schematic are largely adding the correct pin numbers, really trivial.

I have set this up, used xastir, and it was fully working.  To emulate a hardware TNC I used soundmodem, which lets me use my soundcard to encode/decode the data.  I was able to directly solder the components to a DB9 connector, which means they can hide in the DB9 hood.  I also have to use a USB->Serial converter, which is a PL2303 based chip (the most common serial/usb chip).  With my eeepc and this dongle I was more than able to generate the requisite power to switch the PTT, and my desktop radio talked to my VX8R HT (built in TNC).