> cat x.py
class X(object):
    def __init__(self, x):
        self.x = x
        print 'creating X(%r)' % x

    def __del__(self):
        print 'deleting X(%r)' % self.x

class A(object):
    x = X('new')

class B:
    x = X('old')
> python x.py
creating X('new')
creating X('old')
deleting X('old')

Python 2.4.2 (#2, Jan 13 2006, 12:00:38)
Python 2.6a0 (trunk:51513M, Aug 23 2006, 14:17:11)