Personally speaking, I would use the old hidden div trick instead, because of the proliferation of pop-up blockers (which also block interstitials) these days.
It is not actually another window like a popup/under is, but it is rather an HTML container element (usualy a div) that has been made to look like a window (doesn't have to, though, you can make it look like whatever you want). The reason for doing it that way is because that way, even if a user has a pop-up blocker in action, they still receive the content, because the container element is actually part of the same page, rather than a new page (Cf. the little box thing in the top-right corner of Yahoo! member pages).