Well, I didn't see the Mamma Mia movie but regarding both musicals, the one that makes a little sense is the one with the ABBA songs.
As much as I love Queen and enjoyed the experience of seeing WWRY twice (once in London 2002, once in Köln 2006) the "story" is stupid and it's not THAT funny.
IF a movie gets made, some major re-writing would be required and they would struggle to get a decent director to look at it. I wouldn't be surprised to hear both McG and Uwe Boll are interested in the project, hehe :).
Last week Brian said in a radio show that the movie will be directed by Ben Elton who also wrote the sequel. He also said that they are lucky enough to have Robert De Niro in the boat, so the realisation of the movie is probably already quite far progressed.
Musical theatre is everything about live performing in spite of how good or bad the story is (that's why a lot of operas are still being played nowadays). The cinema is totally the opposite: you need a strong story in order to get away with it. It's not that it has to be a wonderful story, but it has to be credible even if it's a fantasy.
I don't believe an idiotic story about Brian May's guitar buried below Wembley stadium and saving a music-less world will have that much staying power to be something more than a vehicle to earn some easy bucks.
Dear god, no... Please may this not be true! I actually like the idea of a movie about Freddie Mercury (with Johnny Depp playing the lead role), but anything to do with WWRY is just... *YAWN*.