Re: if you really want utf-8 headers...

> A simple clean gateways point between old and new could be to
> define that protocols over IPv6 only use UTF-8. 

this is a really lousy idea, unless you want to make BOTH transitions
(IPv4 -> IPv6 and ascii -> utf-8) more difficult.  it's far easier to 
upgrade one part of the system at a time than to require that you have
to upgrade all of your MTAs, MUAs, gateways, firewalls, etc., at the 
same time you introduce IPv6.