I think a raw format like HTML and/or XML would be good, but I also like the idea of a PDF file. I think PDF files will be more friendly to most end users, although being able to view an HTML document in a web browser would also be fine. But like Matt said, it is made more complex by the fact that you have conversations now and not just individual messages. Would be great if, say, an index of all the archive conversations could be made at the top of hte file (which looks like your inbox) and then when you click on one of the conversations, it would jump down to that section in the document (which could easily be done with HTML and PDF files).
..Al