Instance variables declared in ObjC implementation file

This is indeed a new language feature, and if you must declare your ivars (rather than simply declaring properties and letting the compiler generate ivars for you) it's a good practice. Your header files in theory should only expose public interface for your classes; everything else belongs in the implementation.

One caveat is that implementation-file ivars are not visible to subclasses, which can occasionally be a little bit awkward if you have manually generated setters and getters that you need to subclass.

Declaring iVars inside the implementation is definately a new construct in objective C. You need to be using xcode4.2 and have the LLVM compiler selected in your build settings. The idea is to keep your header files cleaner. You can list your ivars inside curly braces like this example;

@implementation MyClass {    
  int var1;
  int var2;
}

The answer given by Rahul is not really correct, although you can delare variables in the way he says they would be looked upon as static by the compiler. Probably for the cases in which he used them it didnt matter.