This network drive is actually mapped through the VirtualBox guest additions. Under Python 2.7 (official 64-bit MSI installer), this works fine:

>>> s = 'Z:\\__svn__\\Lib\\test\\keycert.pem'
>>> os.stat(s)
nt.stat_result(st_mode=33206, st_ino=0L, st_dev=0, st_nlink=0, st_uid=0, st_gid=0, st_size=1783L, st_atime=1287771307L, st_mtime=1286578916L, st_ctime=1286578916L)

Under a freshly compiled 32-bit py3k, though, it fails with a rather strange error message:

>>> s = 'Z:\\__svn__\\Lib\\test\\keycert.pem'
>>> os.stat(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
WindowsError: [Error 1] Incorrect function: 'Z:\\__svn__\\Lib\\test\\keycert.pem'
>>> errno.errorcode[1]
'EPERM'

While a local directory works fine:

>>> os.stat('c:\\Windows')
nt.stat_result(st_mode=16895, st_ino=0, st_dev=0, st_nlink=0, st_uid=0, st_gid=0, st_size=16384, st_atime=1287843075, st_mtime=1287843075, st_ctime=1247541608)