As I stated in response to Georg's other question, I'm not sure how to resolve the difference between common terminology and precisely defined tags. The iPhone is a weird case where the term "iPhone development" has come to mean a specific SDK, unlike how "Mac development" or "Windows development" could mean several different things. Therefore, when someone comes here with a question about the iPhone SDK, they almost always tag the question [iphone] and rarely anything else.
Likewise, among the developers I know we use Cocoa Touch and iPhone SDK synonymously. In fact, I've been using the term Cocoa Touch more lately with the addition of the iPad, the rise of the iPod touch as a platform, and the name change of the OS to iOS.
Is this precise? No, as Georg points out. The iPhone SDK has non-Cocoa elements in it, like OpenGL ES and OpenAL, so it's not technically all Cocoa Touch. That doesn't mean I still won't call it Cocoa Touch, because people will know what I'm describing.
I guess the question is, do we go through the effort of retagging all 38,000 [iphone] questions to use a more precise taxonomy and then instruct new askers on the proper tagging, or do we just adapt to how developers use terms in practice? This isn't a very good answer to your question, because I don't know the proper direction myself.