Ever changing SDK issue

No. You can ignore betas, and you can ignore new OS versions until the one you use doesn't work on the latest device anymore.

And if you have done any object oriented programming before, and if you can read a manual, then learning Objective C doesn't take long.

 

Core APIs don't change very often in any good environment. For example, until 10.5 I could compile and run a lot of NeXTStep/OpenStep software from the GNUStep site on my Mac. Most developers won't ever use every facet of the SDK's class library, but if they do, they only use some parts once.

 

New things tend to be added faster than old things are removed. So, while you lose out on using new features if you don't lose the new things, the knowledge you have tends to stay valid for quite a while. For example, I write and sell an app that I started writing around the time OS X 10.4 Tiger came out (2005). The latest version still runs on Tiger. Nothing it takes advantage of is unavailable in Lion, so it runs on Lion just fine too.

It's often fun to learn and use new APIs. Once you're comfortable with Objective-C and Cocoa, you'll find that it's not really that difficult to keep up.