+1.
I'm getting up to speed with Cocoa to the point where I might be faster than a designer with Photoshop... as long as we don't venture into wooden textures. :)
I think there's a difference between using Photoshop for exploratory purposes, and using Photoshop to produce a deliverable.
I know Chris Coyier is a fan of using photoshop to explore a little bit, before you get into the mode of writing code, which he thinks can interfere with your creative process.