The N1MM contest logging program can use the computer's sound card as a digital voice keyer. This project shows how to interface the computer to a Kenwood TS-2000 to take advantage of this nifty feature. The project uses the Ultimate Linking Interface (ULI) device, but just about any other rig interface can be used.
It looks like using a higher end rig interface will greatly simplify the wiring by reducing the number of cables. For example, the ULI requires two COM ports, two sound card cables, a LPT port and an extension cable for a microphone. To find a better solution, I reviewed several other rig interfaces and decided the MicroKeyer ($300) from MicroHam seems to be the best of breed.
My wiki holds the project documentation.