iPhone Beta 6 Software Development Kit released
May 29, 2008

Apple has released its latest beta 6 SDK for the iPhone. This new SDK comes with some bug fixes and support for the new Leopard 10.5.3 OS. You will have to update your operating system to Leopard 10.5.3 before installing the new SDK.
The Leopard 10.5.3 OS was also released recently which also comes with a number of bug fixes and introduction of some new features such as built-in contact syncing with Google Accounts and Gmail. Read more about it on ArsTechnica
Source: Into Mobile