Running Python 3.1/ IDLE, which was installed on top of a Python 2.5.4
install, Mac OSX 10.4

This seems like such an obvious bug, but I can't find it in the current
list of issues - so I suspect that it may not be reproducible on other
computers, but it's certainly reproducible on my laptop.

If I run a module with (any?) syntax error (for example,

for i in range(10) #missing semicolon
    print i

the interpreter will catch it and send you to fix it. Then, any
subsequent attempts to run that same module will freeze IDLE. The
problem doesn't occur if you run a different module.

I can't reproduce a hang with the python.org 3.1 IDLE on either 10.4 or 
10.5 and with the system Tcl/Tk or with a newer Active Tcl/Tk 8.4 
installed.  Do you have another version of Tcl/Tk installed in 
/Library/Frameworks?  Can you give a step-by-step procedure for 
reproducing the problem you see?

I have Tcl/tk 8.4.7 installed.

To reproduce the hang on my machine:
open IDLE
new window
enter the following code:
  for i in range(10)
      print(i)
run module (saved as test.py)
interpreter complains (shell is still responsive at this point)
fix the code by adding the colon after the for loop
run module again (at this point, IDLE hangs)
While the top menu bar is responsive, all options are greyed out, and
apple-Q doesn't work. Force-quit is necessary to shut IDLE down. Also,
the force-quit menu doesn't show IDLE as being unresponsive, whereas
usually there will be a red alert for an unresponsive program.

Hope this helps

Thanks, I'm now able to reproduce on both 10.4 and 10.5 with the Apple-
supplied Tcl/Tk 8.4.7 but apparently not with a newer Tcl/Tk nor with 
2.6.2.  Investigating further.

This issue was reported again in Issue9763; at the time, I overlooked your original report.  As there is a more recent discussion of it over there, I am going to close this as a duplicate and add you to the nosy list there.

For the record: The problem was caused by a minor change in IDLE during the port of IDLE from 2.x to 3.x.  It is fixed by the changes for Issue11053: r88234 (3.2rc2) and r88242 (planned for 3.1.4).