I'd like to bring up how cross platform the different suggestions you've received so far are:
Sprite Kit: iOS and OS X
Cocos2D: Everything supported by Sprite Kit + Android, Linux, Windows, Windows Mobile, Blackberry and Web Browser
Unity: Everything supported by Cocos2D + Wii, Wii U, PlayStation 3 and Xbox 360.
Sprite Kit looks interesting, but I'm using Unity because:
1 - Although my gameplay mechanics are all 2D, I'm thinking about making the art be 3D. If I'm going to want it to look nice, I'm going to have to make 3D models and produce sprite sheets from those to make a good looking 2D game - why not just take those 3D models and stick them right in my game (there are reasons, IE, I'll need to think about lighting and stuff if I don't go the pre-rendered approach, plus pre-rendered is faster. But it's a possibility and I'm not closing off the option until the game is much closer to complete.)
2 - If my game ends up being successful on Mobile and Desktop, I might want to port it to Consoles as well (the entry fees are a lot higher for those, but how freaking cool would it be to say you've made a game available for consoles?) If I use Unity to start with, this should be a near non-issue... about the only change I'll need to make is... actually, none, since I'm having joystick support anyways, just for incase the user has a controller for iOS or their computer. I might have to figure out how to make it work without an X/Y cursor for the 360 and PS3, since neither of them have anything standard (unlike touch for Wii U and mobile, mouse or trackpad for computers, or IR pointer for Wii.)