The problem does not happen with Python 2.6.
The difference is that:

* in Python 2.6,  the subprocess module would try to write at most 512 bytes
cf L1221 in subprocess.py
chunk = input[input_offset : input_offset + 512]

* in Python 2.7 and py3k, the subprocess module will try to write at most _PIPE_BUF bytes
cf L1319 in subprocess.py
chunk = input[input_offset : input_offset + _PIPE_BUF]

When forcing PIPE_BUF to 512 in selectmodule.c, the test will pass.
PIPE_BUF seems to be broken on AIX.