This is a duplicate of Issue6864. The problem is seen with the Apple-supplied IDLE in OS X 10.6 and appears to be an interaction with IDLE 2.6 and the Apple-supplied Cocoa Tk 8.5. One workaround is to install Python 2.6.5 using the python.org OS X installer and use its IDLE which does not exhibit this problem because it is linked with Tk 8.4. Or, if you can use Python 2.5 instead, you may be OK with Apple's built-in /usr/bin/idle2.5 .
Idle provided by built-in python in Mac OS X Snow leopard i.e. python 2.6.1 is running without any issues. It is /usr/bin/idle2.6. And if by very new mac you mean latest intel based macs released last summer, then yes I have the same machine.
Shashwat: what happens when you launch /usr/bin/idle2.6 and then open a new window in it (CMD N)? My experience is like Aaron's (and many other reports): the Apple-supplied idle2.6 in OS X 10.6 usually hangs after opening a second window, requiring a Force Quit. The problem appears to be pretty specific to the combination of idle2.6 and the 10.6 Apple-supplied Tk 8.5. It's not a problem with the python.org 2.6 which links to Tk 8.4.
Ned : After I opened a new window, that is how it opened : http://imagebin.us/images/gbg8zdr25hviw26hnqfv.png And IDLE hanged a little later. Had to do a force quit. So yes, the problem is in Apple build Idle2.6