Hi,
An update on libpd in unity, it's working on unity free: https://github.com/patricksebastien/libpd4unity
You need to copy libpdcsharp.dll where Unity.exe is installed or where your game executable is located in case of a pc standalone build.
A very short video (watch out: loud) showing a theremin connected in libpd using comport and sending the data to unity.
http://www.youtube.com/watch?v=fVlGEmsO0Ps
I need to learn how to play the theremin :)