Apple changed the name to iPad Air because they will next launch an iPad Pro
Think about it. It makes sense. The iPad is eventually going to be able to do everything the Mac does and will one day replace it. The Mac has the Air and the Pro lines for different demographics. So why not the iPad? This streamlines the naming convention and brings the iPad in with the Mac line.
I bet in a year or two, Apple plan to release an iPad 'Pro' with a >12" screen aimed at productivity minded individuals. In fact today's name change gives credence that the rumors of a bigger iPad is indeed true.