Apple late yesterday published a second beta release of the iPhone 3.0 SDK, available to paying iPhone Developer Program members.
The SDK update is primarily a bug fix release, though some developers report advanced accessory support. The iPhone OS 3.0 will open up access to third-party accessory communication, via both Bluetooth and the 30-pin iPod Dock Connector port.
The update also comes with an updated iPhone OS 3.0 build for all Apple Mobile Devices. The update addresses stability issues introduced in the first beta.