I managed to get 3DS Max communicating with 3dsmax the other day. It uses MaxScript and .NET to communicate over the serial port. It's just morphing a face in this demo but it could really be used to control anything in Max.
It's still got a delay between when you spin the potentiometer and when it's processed in max. I'm not sure if the issue is on the Arduino sending the data or in Max taking time to process it. The weird part is the LED's which are controlled by Max react almost instantly, even though it's still in the process of slowly receiving the potentiometer data. If anyone could guess as to what it is, please let me know.