Thanks for the replies!
A good question to ask is whether you see mobile dev on the horizon for yourself?
Possibly, but for now It's Win/Linux/OSX. I have experience creating mobile games (iOS using Cocos2D, so the hardware level is removed somewhat, therefore I was not fully aware of the situation there).
I think you're asking the wrong questions though. You should first ask "what do I want my platformer to look like?". This will then help you decide which 3d card features you do or don't need.
Look/Style wise, think Braid. Simple Platformer, relatively simple art style. I don't foresee needing any wacky Fullscreen Postprocessing etc (although I guess having the option is always worth it).
The main issue I have is that I created a 3D engine several months back that used GL3.1 and the extra work to manage everything (Matrices, Shaders for EVERY object) seemed such overkill (especially so for 2D), although perhaps this wouldn't be such an issue when I remove a dimension.
(As Evan points out, if you write to OpenGL 3 spec you are basically writing OpenGL ES 2.x.)
So are OpenGL ES 2.X and OpenGL 3.1+ Literally exchangeable / no changes required ?
Thanks in advance!