The SDK is available for Windows, OSX and Linux. "The first public release of the Mojo SDK is an exciting moment for the webOS team, and we hope it is for you too. We know that a thriving ecosystem is critical to the success of webOS and getting the SDK into your hands is just the first step. We think we have something truly game changing with webOS. Join us in what we expect will be an exciting rewarding journey. — Mike Abbott"
The site seems to include pretty thorough information, including a Developer Guide, and User Interface Guidelines. The SDK is available for Windows, OSX and Linux."