I've started to read all available apple docs on this topic. A fast overview gave me a number of about 8.000 (eight thousand) DIN A4 PDF pages. About 10% of the content is almost equal for my feeling. After reading for 2 weeks now, I figured out that I would forget fast what I have been reading a week ago. Unless I start to do something with the knowledge, it's like a drain in the brain.
What are your learning strategies?
write the code as you read/lean about it (like hello world, kind of things). That is the only way I can seem to retain things.
When I am learning any new language I take the provided sample code and rewrite it from scratch. That way if I get stuck on how to do something I can flip over to the provided sample as a reference. I find simply rewriting sample code helps me, especially when there are big syntax differences.
With regards to the iPhone, I would play close attention to memory management. There is some great sample code and documentation in the iPhone Dev site.