My own prediction is that the Avengers will gross at *least* into the 800M range WW.
-It has RDJ as Tony Stark. That alone is a huge draw, that the marketing has emphasized. Basically, I see everyone who went to see IM1 and IM2 going to see Avengers.
-It has Captain America and Thor, both of whom had moderately successful movies. There's at least *some* non-crossover audience to be had.
-It has obvious and marketed "big explosions" summer blockbuster elements, which should draw in the general summer blockbuster audience more than, say, Thor did.
-The latest trailer set a new record for views online, which suggests definite interest.
-Strong positioning of release, with relatively little real competition.
-Growing international market.
-3D bump.
All these reasons combined make me think that, at a bare minimum, Avengers will manage 20% better gross than Iron Man 2. Now, how high it could go? I think a billion is not certain, but its not out of the question.