Touch and hold for a second or two, that is. It doesn't happen immediately. A yellow square shows up around your finger to show that the focus and light adjustment areas have change (from the center) to where you want them.
BTW, it's not the picture itself you are selecting but an area of the camera sensor. If you set the focus in the corner it stays their until the next picture, even if you move the camera to a different view.
I've no idea if the iPad 2's camera app can do this. If it can't, a third party camera app should work.
Speaking of which, some third party camera apps let you set the focus and light metering to two different locations. That could be handy for tricky shots.