Oh, damn, apparently, it has been discovered how do you unlock costumes. It's a big twist, I wasn't expecting it. This has been discovered in the Official Strategy Guide of the game.
So, you have to take photos of some spider symbols that are located around the city. The thing is that those logos AREN'T THERE at launch. More logos are going to get available as the weeks advance past the launch day of the game, being introduced in the game by an online update (invisible online update, supposedly).
That's the reason why eveyone who has the game only unlocked the cross-species suit (available after beating the game) and the Kaine's Scarlet Spider suit (this weeks costume). Also available right now is the SM3's Black suit, which is available after beating the 100% of the game.
So, the game needs internet connection at least one a week so you can "download" automatically those new spider symbols, so you can obtain the new suits. If not, you are confined to the two movie suits (belt and sans belt), the cross-species suit and the SM3's Raimi's suit. Forevah.
Of course, this "DLC" will be FREE.
PS: Knowing this, I would have love Arkham City even more if it has this same system instead of paying for the skins.