Sorry, this happened to me in /tmp, where it's actually true, except I 
don't expect symlinks to be considered mountpoints, so I still consider 
it a bug. Should have tested more though.

A fix for the problem is easy. One has to replace os.stat() with os.lstat():

def ismount(path):
    """Test whether a path is a mount point"""
    try:
        s1 = os.stat(path)
        s2 = os.stat(join(path, '..'))
    except os.error:
        return False # It doesn't exist -- so not a mount point :-)
    dev1 = s1.st_dev
    dev2 = s2.st_dev
    if dev1 != dev2:
        return True     # path/.. on a different device as path
    ino1 = s1.st_ino
    ino2 = s2.st_ino
    if ino1 == ino2:
        return True     # path/.. is the same i-node as path
    return False

Go ahead then!

Fixed in r59709 (trunk) and r59710 (2.5)