This is how I hang the intepreter in 2.4a1:

python2.4
...
>>> import readline
>>> import rlcompleter
>>> import thread
>>> thread.start_new_thread((lambda: 1), ())
>>> ^C
(Python Interpreter hangs)

The ^C of course is the Keyboard interrupt. It doesn't
matter if I write something on the command line and
then press ^C, the interpreter still hangs.

This is new. The interpreter won't hang under Python
2.3.4; it won't hang under Python 2.4a0.

It specifically changed sometime thursday and friday
(Python 2.4a0 -> Python 2.4a1). This might indicate the
change logged as: "Non-main threads no longer have all
signals masked." or the change logged as: "A different
interface to readline is used." is to blame.

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I forgot to say: I'm using Mac OS X 10.3, fink project readline.

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Confirmed on linux/x86.

Hmm, I see the problem (my lack of understanding of the
Python thread API).  I have a fix (just running make test)
but I don't know what the score is wrt 2.4a1...

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Well, 2.4a1 is already out the door (well, almost), so any
fix will have to go into 2.4a2. Feel free to post a patch
here as well, though, so we can point the hordes of testers
who discover this bug at this patch.

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Here's the patch that just got checked in as 

Modules/readline.c revision 2.72
Parser/myreadline.c revision 2.33

Should go onto /2.4/bugs.html when that exists...