Microsoft provides a library for accessing files with the Open XML format (this is the format that Office 2007 uses). The System.IO.Packaging namespace (part of .Net 3.0) already allows you to open these packaged files but the new SDK goes a lot further. It is built on top of the System.Packaging API and provides strongly type classes to manipulate Open XML documents.
If you have questions about the SDK you can use the Microsoft forums. Another interesting post about it is the one from Erika which has collected a number of links about how to programmatically create Office 2007 documents. Wouter also explains that the main purpose of the new SDK is to provide a higher-level abstraction of the Open Packaging Convention.
PS: You might also want to take a look at So what's the fuss about the new Open XML file formats?