Paul Woods, designer at ESPI, in The Power of Keynote:

Keynote is an incredibly powerful design tool. Less then one year later, I now rarely (if ever) use InDesign to layout presentations, and I have started using Keynote almost exclusively for any web layouts I do. And not just UX wireframes, but full UI designs. On a recent project, I also used it for poster mock ups, banner designs and a bunch of other formats.

It seems like a great replacement for InDesign, but no Photoshop yet:

Also, I really miss things like graphic styles and proper tools for drawing vectors (there is a Bezier tool, but it is very limited). Another issue is the lack of detailed kerning options, although to be fair, the typographic tools in general are good. Some basic blending options (Multiply, Overlay, etc.) would be the proverbial icing on the cake.