1) Even if The Hobbit started filming right now, right this second, they wouldn't be done by next winter. There's just no way. Remember, they are making two huge three hour movies at the same time. Its the Lord of the Rings strategy. Film them all at once. And then they'd still have all the VFX work to do. So I think Superman will be safe and sound next December.
2) Even if they were to come out at the same time and one of them had to move, Superman would be the one pushed out of December. The Hobbit is seen as a Christmastime movie. Hell, it was my Christmas tradition for four years in a row. Watch the movie one year, get the extended edition for Christmas the next year. Ah, good times...
With Superman, however, and the superhero movie genre in general, its seen as a summer type of movie. So if WB were forced to rethink their strategy, I'd think they'd get Superman into the summer slot.
3) Even if, after all that, they were forced to open within the same month time frame, so long as Superman is good it shouldn't really matter. "Oh, but Pirates..." No. Wrong. Sorry. People always say that. Pirates hurt Superman. No, says I. Pirates did not hurt Superman. Superman helped Pirates. The poor quality of Superman Returns meant that there wasn't any other quality entertainment out...until Pirates, with its wacky protagonist and huge action set pieces and gallons and gallons of spectacle appeared. Pirates was nothing but pure, nonstop summer entertainment for two and a half hours. So why does nobody ever look at it the other way? Couldn't it be that the dour and dull quality of Superman Returns allowed the more fun movie to do better than it would with a more exciting movie released next to it?
But what if they did both have lots of action and CGI and spectacle and what have you? Well, remember summer of 2007. Spider-Man 3. Pirates 3. Shrek 3. All released around the same time. And they are still the three highest grossing movies of that year. Despite their somewhat dubious quality and reputations, they proved that three big huge movies can co-exist within the same month.
So really, just think of it this way. If Superman is good it should be fine, no matter what competition it is up against.