I need a full featured high volume email and newsletter system capable
of handling dynamic content assembly by multiple users within the same
template.  Users would log in and create their message for their part
of the message or newsletter.

The system must be full featured including, but not limited to,
subscriber management for multiple lists including spitting and
merging mailing lists, automated bounce and opt-out processing,
content management with library for templates and images, statistics
and tracking with reports, and scheduled delivery to automatically
send emails at scheduled times.

As you are basically asking for a Mailing List manager combined with
CRM features, there will most likely not be such a tool. There are
various tools to handle the mailing aspect, such as www.sympa.org or
LISTAR. For the other functionalities you will most likley end up
coding your own front-end

You are probably right, but I want to make sure before going to the
trouble.  To answer your question, I need for users to be able to log
in and create their individual messages which are then combined into
an email and sent to subscribers.  The rest is just regular email
stuff.

I've been using a program called ebizmailer from
http://www.zustek.com/home.htm for several years now. It supports most
of what you are looking for, or can be easily modified to support it.
It also has an email approval system built in. This isn't a cheap
solution it meant for enterprise size business.