Apple has just dropped the sixth (6th!) update to their iPhone SDK and 2.0 beta firmware. Pre-req is Apple's almost simultaneously dropped OS X Leopard update, 10.5.3, so developers are forewarned to get that first before beginning the SDK install.
According to TUAW, however, Apple is now carding. Yup, they're checking Application IDs at the door, so make sure you're registered before you try to install apps on a test iPhone.
Head on over to Apple's iPhone DevCenter to get yours now!
A New Version of the iPhone SDK is Now Available Sixth beta version just posted.