On Fedora 8 and 10 using Python 2.5.1 and 2.5.2 (64 bit):

$ grep nfsnobody /etc/passwd
nfsnobody:x:4294967294:4294967294:Anonymous NFS
User:/var/lib/nfs:/sbin/nologin

So the UID of nfsnobody is 4294967294 (-2 if viewed as signed 32-bit int).

>>> import pwd, os
>>> print pwd.getpwnam('nfsnobody').pw_uid
4294967294
>>> os.chown('some file', pwd.getpwnam('nfsnobody').pw_uid,
pwd.getpwnam('nfsnobody').pw_gid)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: signed integer is greater than maximum

The reason for this error is that os.chown uses the "i" format to
convert the second and third arguments.  But the valued do not fit in a
32-bit signed integer.  uid_t and gid_t are defined as unsigned
quantities on this system.

The bug does not occur on 32 bit Fedora since there the uid and gid of
nfsnobody are 65534.