Hi Patrick,
While we no longer will publish the SDK as a download for InfoPath 2007, essentially all of the documentation from the SDK has been updated and incorporated into the current InfoPath Developer Reference, which is available from the InfoPath form designer and here on MSDN:
http://msdn2.microsoft.com/en-us/library/bb230129.aspx. However, the Developer Sample Forms have not been updated, and consequently none of that documentation has been carried forward.
Also, there is a new set of documentation specfically for managed code called the "InfoPath Developer Reference for Managed Code", which is available here:
http://msdn2.microsoft.com/en-us/library/aa941030(VS.80).aspx. This documents the new Microsoft.Office.InfoPath namespace, and also incorporates and updates the content that was originally published with the InfoPath Toolkit. The InfoPath Managed Code Form Template portion of this documentation is also available from Visual Studio Tools for Applications (VSTA) when you install InfoPath 2007.