Just to add to this thread; in Leopard importing a Finder folder containing a webarchive doesn’t import the webarchive. Everything else imports OK (except Mellel docs - but that’s another story 
) However, dragging webarchives straight into DTPO works fine. All the webarchives in question were saved in Tiger, but I couldn’t say whether it was in the Safari 3 beta era.
Are web archives enabled in the Preferences > Import pane?
Now I see … having just loading on a shiny new install, I went through and clicked off “Markup languages” and didn’t read the rest. Thanks.