FWIW, I'm an auto-didact - everything I know about computers, programming, etc. I learned from my own private reading. So it can be done. I wouldn't presume to be able to, say, write an entire operating system by myself or anything, but for writing end-user apps, you can definitely get good enough to make some pretty nice stuff if you put an effort into it.
My recommendation: Start with a good book on C. From there, get a good book on Objective-C and Cocoa (Hillegass's book seems to get recommended a lot). I haven't studied the iPhone's SDK much, but it seems like it's based on Cocoa, so if you get started by learning Cocoa you should be in pretty good shape, I think.