> /var/mail/username/[individual_mailfiles]
> 
> This can be done in a way that requires no file locking (big plus) and
> can finally eliminate the goddamn From kludge (big plus).

How about Solaris's approach to the "From" problem ?
(i.e. Content-Length: header)

Though I'm not sure that whether it is good choice or not.
--
soda