Please get a grip, this is pretty insignificant compared to the problems a lot of people have in their lives. A bit disappointing? Maybe. A bit inconvenient? Maybe, if you decide to take it back / send it back to get a replacement. Might cause a problem with the screen over time? Maybe, but that's why you have a warranty.
Mine will do it but not in normal use (with or without case) and I have to actually try to flex it / squeeze it with more than normal gripping strength to get it to do it: I'd never have seen it if it wasn't for this thread. If it ever causes a problem (which I think is pretty unlikely) I'm sure Apple will exchange it.
Where exactly in the back is it that people are tapping, screenie?