I haven't done this but wouldn't that just entail setting unique
permissions on the document?

I haven't done it in a workflow, but I've got a Blog entry that shows
the code to do it in an event handler. You could use similar code in a
Visual Studio workflow  code activity to do what you want. The Blog is
located here:
www.mindsharpblogs.com/.../1754.aspx

I would think that you would have to leverage the WSS Object model to
modify the permissions  of an item.

you could certainly do that, but what is required is that if you have person  a,
person b, person c in a workflow  and person a is first in the work flow and
edits the document, once they approve the document  the workflow would change
their permissions  to read-only and then move on to person b, where the same
thing is going to occur and then person c. At that point  the document is no
longer editable by anyone but the site owner. This is requested for content
management purposes.

We want a static copy of the document at the end of the workflow to live in the
records center and a second copy that is as locked down as it can be to live in
the collaboration space as reference. Steve