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I'm not sure if the current behavior should be maintained or not, but it 
looks like to me that the readline module has always generated a 
KeyboardInterrupt, regardless of whether SIGINT has been overridden. 
This is a bit odd though. It causes the SIGINT handling to change 
depending on whether or not you are at the top level interpreter's 
prompt.

wantarray% cat /tmp/foo.py 
import signal

def foo(sig, frame):
  print "caught foo"

signal.signal(signal.SIGINT, foo)


wantarray% python -i /tmp/foo.py
>>> foo
<function foo at 0x61430>
>>> ^C
KeyboardInterrupt
>>> while 1:
...   pass
... 
^Ccaught foo
^Ccaught foo
^Ccaught foo
^Ccaught foo
^\zsh: quit       python -i /tmp/foo.py