Is it possible to sharpen just certain areas of an image?

I don't use apeture but I am sure it  is possible.

It is certainly very easy in LR4 and I could not imagine that it would not be very different for Aperture.

To not be able to exclude sharpening for the sky would be a major reason not to use the software.  For me it would be a pre-requisite for any photograph processing software.  And I understand that Aperture is a good product so if it omits the ability to exclude sharpening of the sky I would be very surprised.