Mailman isn't resigning it.  Should it be?  Does other mailing list software? 

Mailman must take responsibility for the mail itself. It doesn't have to actually sign with DKIM to do so: for backwards compatibility, spam filters fall back to other heuristics to try and figure out the 'owner' of the mail if it doesn't use DKIM. Those heuristics can go wrong of course. Ideally all mail would be DKIM signed. There's no reason not to do it, really.

Yes mailing lists that edit people's emails resign. For example, from a recent message to the bitcoinj list

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=googlegroups.com; s=20120806;
        h=to:from:subject:date:lines:message-id:references:mime-version
         :content-type:user-agent:in-reply-to:x-original-sender
         :x-original-authentication-results:reply-to:precedence:mailing-list
         :list-id:list-post:list-help:list-archive:sender:list-subscribe
         :list-unsubscribe;
 
I suppose it is somewhat acceptable for us to remove subject tags and footers if we have no choice...

Good email clients can extract the same information from the headers anyway. I filter all my mail based on them, and the headers also contain unsubscribe instructions. Gmail is capable of using them programmatically.