to answer my own question, it's described here - http://about.digg.com/diggbar - and seems to be something that's completely optional (you can enter a url like http://digg.com/http://google.com that displays, in this case, google in a frame with a "bar" at the top).

given all that i still don't understand a comment on this page about disabling it - it seems to be something you have to enable not disable...