$ cat test.py
import sys
print sys.stdin.readline()
$ python test.py | python test.py
Traceback (most recent call last):
  File "test.py", line 2, in ?
    print sys.stdin.readline()
IOError: [Errno 35] Resource temporarily unavailable

This happens about 3 times out of 5.
It happens under FreeBSD 5.1-RELEASE (SMP) and:
python-2.1.3
python-2.2.3
python-2.3.2
python-2.3.3c1

It does _not_ happen under Debian 3.0 or 
FreeBSD 4.7-RELEASE (SMP).

According to read(2) this "error" should occur only when 
the descriptor is in non-blocking mode. However as you 
can see from the program it isn't. Further I have caught 
the IOError and run fcntl(F_GETFL) on sys.stdin.fileno() 
and the only flag set is O_RDWR.