Two small clues.

First, a backtrace:

#0  0xb7df102a in strcmp () from /lib/tls/i686/cmov/libc.so.6
#1  0x0809e678 in warn_explicit (category=0x81dd140, message=0xb7ac58f4,
filename=0xb7acced0, lineno=1, module=0xb7f53300,
    registry=0xb7ac9e94, sourceline=0x0) at Python/_warnings.c:393
#2  0x0809f1df in do_warn (message=0x81fbd78, category=0x81dd140,
stack_level=1) at Python/_warnings.c:606
#3  0x0809f37d in warnings_warn (self=0xb7aceab4, args=0xb7af0a7c,
kwds=0x0) at Python/_warnings.c:628
#4  0x081624ee in PyCFunction_Call (func=0xb7acace4, arg=0xb7af0a7c,
kw=0x0) at Objects/methodobject.c:84
#5  0x080b3633 in call_function (pp_stack=0xbfd51f44, oparg=1) at
Python/ceval.c:3403
#6  0x080ae776 in PyEval_EvalFrameEx (f=0x82b5e6c, throwflag=0) at
Python/ceval.c:2205
#7  0x080b1ac8 in PyEval_EvalCodeEx (co=0xb7ade988, globals=0xb7f4f5d4,
locals=0xb7f4f5d4, args=0x0, argcount=0, kws=0x0,
    kwcount=0, defs=0x0, defcount=0, kwdefs=0x0, closure=0x0) at
Python/ceval.c:2840
#8  0x080a686f in PyEval_EvalCode (co=0xb7ade988, globals=0xb7f4f5d4,
locals=0xb7f4f5d4) at Python/ceval.c:519
#9  0x080df486 in run_mod (mod=0x82ba910, filename=0x81a09e4 "<stdin>",
globals=0xb7f4f5d4, locals=0xb7f4f5d4,
    flags=0xbfd52370, arena=0x8216df8) at Python/pythonrun.c:1553
#10 0x080dd67e in PyRun_InteractiveOneFlags (fp=0xb7ec7440,
filename=0x81a09e4 "<stdin>", flags=0xbfd52370)
    at Python/pythonrun.c:958
#11 0x080dd1e0 in PyRun_InteractiveLoopFlags (fp=0xb7ec7440,
filename=0x81a09e4 "<stdin>", flags=0xbfd52370)
    at Python/pythonrun.c:870
#12 0x080dd038 in PyRun_AnyFileExFlags (fp=0xb7ec7440,
filename=0x81a09e4 "<stdin>", closeit=0, flags=0xbfd52370)
    at Python/pythonrun.c:839
#13 0x080ef6ba in Py_Main (argc=1, argv=0xb7f22028) at Modules/main.c:592
#14 0x0805a689 in main (argc=1, argv=0xbfd534c4) at ./Modules/python.c:57


Then, this behavior:
Python 3.0b3+ (py3k:65930M, Aug 21 2008, 21:23:08)
[GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import _warnings
[40709 refs]
>>> _warnings.warn(0)
__main__:1: UserWarning: 0
[40739 refs]
>>> _warnings.warn(12345)
__main__:1: UserWarning: 12345
[40744 refs]
>>> _warnings.warn(AttributeError)
__main__:1: UserWarning: <class 'AttributeError'>
[40750 refs]
>>> import warnings
[41483 refs]
>>> warnings.warn(0)
[41483 refs]
>>> warnings.warn(12345)
[41483 refs]
>>> warnings.warn(10101)
Segmentation fault

That is, _warnings.warn(spam) works OK and avoids the
warnings.warn(spam) crash for values already called by the former.