I changed message.el such that message-default-mail-headers,
message-default-news-headers, and message-default-headers can be a bit 
more flexible.  Specifically, they can be:

    1) A string - the string will be inserted as before
    2) A function - the function will be evaluated, and the string it
                    returns will be used as the headers to be inserted
    3) A list - the list can contain strings and/or functions.  They
                will be evaluated one at a time as in 1 and 2.  The
                results of the evaluation will be concatenated and
                used as headers.

I also took a crack at modifying the customization stuff for the three 
variables, but I don't know how successful I was.

The motivation for this change?  I need some way to insert the output
from a random X-Face function I wrote (see my web page) along with the 
other default headers I set up.

Matt

-- 
     Matt Simmons  -  simmonmt@acm.org  -  http://www.netcom.com/~simmonmt
     Television:  A medium.  So called because it is neither rare nor well
			     done.  --Ernie Kovacs