Default the Archive/Unarchive checkboxes to the current state of the item? For example, if an item is archived, it would be checked by default?

All the ones above the <HR> line are unarchived, the ones below archived. Setting the checkbox makes them move below the line if they are above the line, and above the line if they are below the line.

Have a small message appear at the top of the CB indicating the number of archived items, one that would not appear if there aren't any archived items?

And it does tell you the number of messages in your inbox, already, although it you have more than 10, it indicates the number of archived and unarchived messages. I'm sort of against adding more text, because the point of the display was to be able to keep messages, and reduce the amount of scrolling to get down to the "useful" chatter in the chatterbox (although somedays, no amount of scrolling would help...)