We've always heard that this upcoming Man Of Steel film absolutely had to come out by the end of 2012, due to the fact that significant portions of the rights to the Superman character are due to lapse over to the families of Jerry Siegel and Joe Shuster at the end of that year. The family would theoretically get the rights to the red cape, blue leotard, and his ability to "leap tall buildings," and perhaps character like Lois Lane, while DC would retain most of the villains, including Lex Luthor, and the ability to fly. And seriously, has Superman ever leaped over a tall building? That's a Hulk move. The families would not hold the Superman trademark, which is a pretty crucial part of the rights, so D.C. could theoretically make do without the aspects that they'll be forced to cede to the Siegel/Shuster heirs.
Now it looks like either they might have to do so, or come to some kind of agreement with the heirs, as Man Of Steel has been pushed back to June 2013, a full six months from its previous date at the end of 2012. The reasoning behind this move hasn't been discussed yet publicly (at the Immortals panel at SDCC this weekend, Henry Cavill said explicitly that he couldn't discuss it), which of course leads to all kinds of delicious speculation.
The most obvious theory that's going around the web is that the film simply won't be ready in time; 16 months or so is hardly enough time to make an effects-laden film these days, with Michael Bay complaining that the two years between Transformers films were really cutting it close. Speculation that the script isn't ready yet has been going around Hollywood, although supposedly filming was supposed to start around August or September in Chicago. They might still proceed with that filming, if this delay is related more to effects work than the script.
Another idea might be that Man Of Steel was a bit worried about going up against the launch of The Hobbit, Part I, also due to launch sometime in late 2012, but then, it's difficult to imagine that the competition will be any more kind in summer 2013, with at least Thor 2 and Iron Man 3 also competing for the same dollars.
Either way, this kind of release date shift has to come with a big of egg on the face of DC, which notably failed to make much of a dent in the Marvel hegemony with Green Lantern, which is going to wind up costing Warner Bros. probably something like an even $100 million when you consider marketing costs and distributor cuts. If the delay can prevent something like that from happening to Man Of Steel, then a little egg-face is probably worthwhile for them.