Yes, a SDK is still planned. We always had the SDK in mind during creation of SPSF 2010. A SDK could be used to create your own "SPSF" with you own recipes which can be installed on top of SPSF 2010.
The SDK would work for Sharepoint 2007 as well as SharePoint 2010. Currently we focus our efforts on the final release of SPSF with more recipes and templates.
But be aware that SPSF is published under GNU license. This means that new recipes or templates based on SPSF must also be published to the community.
Our preferred option to bring your recipes and templates into SPSF is to join the SPSF project. Then you could add new recipes directly into SPSF source code at Codeplex.
Or you could send your code samples or code snippets to us and we add them to SPSF.
Everything in SPSF is prepared to value your collaboration in the project e.g. displaying the author of a recipe in the documentation, in the installation helper etc.
What would be your preferred way to extend SPSF?