The following script produces different output on python2.6.6 vs. python3.1.2:
-------------
import subprocess, sys

ls = subprocess.Popen(['ls', '-l', '/etc/motd'], stdout=subprocess.PIPE,)

end_of_pipe = ls.stdout

print('Result:')

for line in end_of_pipe:
    print(line.strip())
-------------

Result from invoking with python2 and python3 are respectively:

Result:
-rw-rw---- 1 root root 25 Jan 18 10:25 /etc/motd

and:

Result:
b'-rw-rw---- 1 root root 25 Jan 18 10:25 /etc/motd'

Is this difference a feature, or a bug ?

It's a feature. Subprocess output is binary data, not text; and since python3, the string type is now what python2 called unicode!

Please read http://docs.python.org/py3k/whatsnew/3.0.html#text-vs-data-instead-of-unicode-vs-8-bit
for an explanation of the most important difference between python2 and python3.