Before starting to develop an iPhone application the programmer must know, in what SDK he is going to work and what is the latest SDK provided by Apple for newer version of iPhone or iPod Touch.
SDK:
A software development kit (SDK or "devkit") is typically a set of development tools that allows a software engineer to create applications for a certain software package, software framework, hardware platform, computer system, video game console, operating system, or similar platform.
The latest SDK available for Iphone application development is iPhone SDK 3.0 .iPhone SDK 3.0 comprises of iPhone development Packages, XCode IDE and many integrated tools like memory leakage measuring, performance measuring and so on.
To get the updated Iphone SDK from Apple one must get registered as a iPhone developer in apple's site.
Here the registration link
After getting registered as iPhone developer you can download the latest SDK from apple server itself. Here is the download link
