Viewing message headers is nigh-on impossible. Well, that's not entirely true. One can see the internet headers of an RFC822 message by visiting the View->Options menu item that's only available when you have the message open in its own window. Viewing the source of an HTML message appears to be flat-out impossible.
You can also right click on the message in the list view and select Options to get that same window. But yeah, that is the suck.