Increasing iPad functionality will eventually eat into Mac sales. Apple can't make the iPad powerful enough to do things a normal computer will do, but they have to iterate to make people keep buying the newer model. I'm interested to see how they go about doing that.
If you look at the technology and form factor of the MacBook Air and the new iPad, you might be lead to believe that in terms of computational power they actually merge in perhaps as little as 2-3 years time. You end up with a tablet that can run Final Cut Pro X on an external display via Thunderbolt, or possibly even wirelessly via AirPlay or some iteration thereof. Launchpad even looks like an iOS launcher for the Mac.
I suspect that Mac OS X and iOS merge at some point, possibly not too far out. It may be a lot sooner than anyone expects. There could even be an iOS 7 / OS X 11 announcement a year from now.