Try using pkg_resources, which is part of setuptools (and available on all of the pythons I have access to right now):
>>> import pkg_resources
>>> pkg_resources.resource_filename(__name__, "foo.config")
'foo.config'
>>> pkg_resources.resource_filename('tempfile', "foo.config")
'/usr/lib/python2.4/foo.config'
There's more discussion about using pkg_resources to get resources on the eggs page and the pkg_resources page.
Also note, where possible it's probably advisable to use pkg_resources.resource_stream or pkg_resources.resource_string because if the package is part of an egg, resource_filename will copy the file to a temporary directory.