Some info on the different USB<-->DC serial approaches:
USB --> Generic USB to serial port converter --> DC Coders Cable --> DC
Should work at 115.2Kb/s.
This would be the out-of-the-box solution.
USB --> Axlen's USB cable --> DC side of the Coders Cable --> DC
Works at ~500Kb/s, unreliable at 1562.5Kb/s
This is the one that was mentioned in this thread. You have to build it yourself and it seems that it is unlikely to work at the high speed with many PCs. But the lower speed version should work fine. Also you need to cut up a regular Coders Cable for the connection to the DC serial port.
USB --> Chaos/jj10odm's USB cable --> DC
Works at 1500Kb/s.
This is the newest development. (Check out http://www.dcemulation.com/phpBB/viewtopic.php?t=78829 starting at the 2nd page). So far only Chaos/jj10odm has built it. I'm actually in the process of building this. Requires you do crack open a DC serial connector from the Coders Cable or VS Link Cable, or connect it directly to the serial port inside the DC.