My retina iPad mini has the same issue. There is little to no play side to side, but there is a fair amount of wiggle back and forth. My original non retina mini had tight buttons all around, so the wiggle power button was a little surprising and disappointing to me.
I tried returning the iPad the day after I got it, but the Mac store would not take it back. They said as long as the actual button was functional, there was nothing they could do.
3 weeks later, I hardly notice the wiggle anymore. I'll gladly take a wiggly button over dead pixels, or image retention.