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