The news you don't expect to read: pop-unders are working! At least in Orbitz' case, as reported today on iMediaconnection.
A few weeks ago eMarketer talked about it as well, saying quite the same thing.
So, the question is: are pop-unders really effective or do Otherwise have a very good PR team? Telling the truth I grant them a point: if you ask me if I remember which ads are popping up (or under) on the New York Times pages, it takes me just one second to name Orbitz.

All right, maybe pop-unders ARE working, but you need to use them massively in order to get the right effect. Furthermore Orbitz ads are something a little bit different, giving the user the chance to have a little bit of fun with them.
Anyhow I'm not convinced about them.