This will probably be the main topic in this forum from now to 2016, but the true is that Rio doesn't need to construct another athletic stadium. Joao Havelange stadium already serves its purpose (for athletics competition, not for ceremonies), building another stadium just to acomodate the olympic tradition is NOT part of Rio's plans. Maracana stadium is being renovated for the 2014 world cup and FIFA rules wouldn't allow an athletic track to be part of the new stadium. So, since the ceromonies' stadium needs to be a big venue, they are going with Maracana. Maybe Rio will have two flames, one for the ceremony and another for the Olympic Park.
By the way, the athletic stadium in Rio will not be located in the Olympic park, another first I guess.
Anyway, I digress. Back to London, I'm glad that we will finally see the flame from now on.