Not really. Just a few things... Webmail, IIRC, and announcement list mail. Mail sent from customer mail machines via SMTP, customer web machines, etc. wouldn't be affected. And it would be fairly trivial to "fix" this problem on the webmail machines and most other places.
This isn't a new problem, and the fix is simple... separate caching-only and authoritative nameservers. This is already the case on most DH machines, and I imagine if you write support with information about this, someone will take care of it.
Be specific about where exactly you're seeing the problem (i.e., webmail, discussion list mail, whatever).
Not really possible to coordinate properly for a seamless transitions. Plus, the registrars for some TLDs check whether a nameserver is responding authoritatively for a zone before letting someone assign the domain to a given nameserver.
I don't think requiring confirmation from the domain's contact address is viable either. Even just the volume of support inquiries and other problems this would generate would be a huge problem.
There are some basic checks in place to prevent people from adding AOL, Hotmail, etc.