There certainly could be differences in behavior considering how old 10.6.8 and 10.4.11 are.  I'm not a subprocess expert but it seems to me that, if your program hangs doing a readline from process.stdout, chances are you are running into a pipe buffer deadlock as warned about in the subprocess module documentation:

"Warning - Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process."

https://docs.python.org/dev/library/subprocess.html#popen-objects