This is because y is a new-style class (because file is) and z is not.
If the return type of new-style classe's len is not an int, Python tries
to convert it, which is why that works for the first example. However,
that conversion doesn't happen in old-style classes. Try:

>>> class z(object):
...     def __init__(self, name):
...         self.f = file(name, 'r')
...     def __len__(self):
...         self.f.seek(0, 2)
...         return self.f.tell()
>>> x = z('/tmp/longfile')
>>> len(x)
[Whatever it is]