It's very excited to know that Microsoft Office SharePoint Server 2007(Beta) SDK is available. You can get the downloadable version or just browse it online on MSDN.

Install version: SharePoint Server 2007: Software Development Kit
MSDN online version: Welcome to the Microsoft Office SharePoint Server 2007 (Beta) SDK

In addition, if you just concern about the WSS, you can get the WSS v3 SDK from Windows SharePoint Services V3: Software Development Kit.