You can browse the source code to the serial proxy at:
http://cvs.berlios.de/cgi-bin/viewcvs.cgi/arduino/serproxy/
Instructions for downloading it are here:
https://developer.berlios.de/cvs/?group_id=3590
but to summarize:
cvs -d:pserver:anonymous@cvs.arduino.berlios.de:/cvsroot/arduino login
cvs -z3 -d:pserver:anonymous@cvs.arduino.berlios.de:/cvsroot/arduino co serproxy