His hair is definitely receding, without a shadow of a doubt.
In my early 20's I had a condition that made some of my hair fall out. Fortunately I got over it and it all grew back, but it did make me take note of hairlines/balding. HC temples show quite clearly, and quite a way back in many of his pics. The official Supes pic released shows clear recession.
Truthfully as sad as this sounds, this was my only worry when I heard he was cast. His face, eyes, expression all scream comic book Superman to me, but the hair...not so much. Whilst I don't think it's a problem for this particular movie, the rate it recedes could really be an issue for potential sequels.
A receding hairline will show maturity, which can be a good thing for our Superman. The flip side to that however, is that there are few things that will age a man quicker than a weak hairline. Given the glacial rate of development and production for this movie, it doesn't bode well. Assuming we get one, the absolute minimum time to get MoS 2 would be 2015. That's plenty of time for him to lose a bit more, but that really depends on his genes.
The thing he has going for him is that it's curly, and that hides a receeding hairline
very well to the point of it not being noticeable form certain angles. Take Perry Cox from scrubs. Even before he started that 10 years ago or more his hairline had recessed very aggressively, however when he grew out the length it looked as though he had a full head of hair.
Infact the pic a few posts above me proves this exact point. Here you would never know that under the curls his hairline hides quite a way back. Slicking back the hair though, there is no hiding it..
I too really think they should leave his hair how it is now, or was at comic con for Supes. It looks full, thick and the curl drops naturally it seems! The issue would be what to do for CK