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I managed to reproduce this with Ubuntu 10.10 and the gcc-snapshot package. I've shrunk it to the following (no floating-point in sight!):
/* file dtoa.c */
static char s0[12];
char *_Py_dg_dtoa()
{
 char *s = s0;
 *s++ = '9';
 *s++ = '9';
 while(*--s == '9')
 if (s == s0) {
 *s = '0';
 break;
 }
 ++*s;
 return s0;
}
int main(void) {
 char *s;
 s = _Py_dg_dtoa();
 return s[0] != '1';
}
/* end file dtoa.c */
dickinsm@ubuntu:~/bug$ /usr/lib/gcc-snapshot/bin/gcc -O0 dtoa.c && ./a.out && echo "Success" || echo "Failure"
Success
dickinsm@ubuntu:~/bug$ /usr/lib/gcc-snapshot/bin/gcc -O1 dtoa.c && ./a.out && echo "Success" || echo "Failure"
Failure