System: Ubuntu 8.10 Desktop AMD 64
Processor: Intel Core i7
Python: 2.5.2

While raw C code:

#include <gnu/libc-version.h>
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
  printf("GNU libc version: %s\n", gnu_get_libc_version());
  return 0;
}

prints:
GNU libc version: 2.8.90

Python ctypes seems to bug:
$ python
Python 2.5.2 (r252:60911, Oct  5 2008, 19:29:17) 
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys, ctypes, ctypes.util
>>> sys.version
'2.5.2 (r252:60911, Oct  5 2008, 19:29:17) \n[GCC 4.3.2]'
>>> sys.platform
'linux2'
>>> LIBC = ctypes.cdll.LoadLibrary(ctypes.util.find_library('c'))
>>> LIBC
<CDLL 'libc.so.6', handle 7f6d83122958 at 7f6d82fe8210>
>>> LIBC.gnu_get_libc_version()
-2109473701

Ooops, my mistake, there is nothing wrong, this report can be closed, sorry.