Just judging from the ads and the trailers I'd say that this movie is definetley pandering to a female audience. Opening the same weekend as The Dark Knight probably isn't the smartest move since TDK is one of, if not the most anticipated movies of the summer even before Heath Ledger's death, which only propelled interest even higher. Dark Knight will certainly open at #1 that weekend, but I guess we'll just have to wait and see how Mamma Mia's box office fares.
I don't know which one I'm going to see first, but it may be TDK since I'm seeing Mamma Mia mainly for Meryl and I'm not a big ABBA fan.
"You drank a charm to kill John Proctor's wife! You drank a charm to kill Goody Proctor!" - Betty Parris to Abigail Williams in Arthur Miller's The Crucible