Looking favorably on List-Id tags is just opening a big bypass in dkim email checking. I really like the propositions that say we're going to get with DKIM and continue on to a brave new internet.
Good practice plan:
1. Encourage List managers to reject email with broken signatures or that conflicts with http://tools.ietf.org/html/draft-ietf-dkim-ssp-03 Author Signing Practices. I think this will deal with the "vouching for the validity" problem of signing.
2. Encourage List managers to DKIM sign content. This is just making a statement that the content came through the server and that you've done DKIM checking on the input.
3. Provide a mechanism for List managers not to mangle DKIM signatures. Provide or refer to "DKIM - A list managers' guide"
4. Encourage DKIM implementers to have a policy of configuring favorable DKIM signatures that will allow an email though even if the Author Signing Practices fail. This is mainly for those List managers that insist on breaking signatures.