This is pretty cool... just announced at the TechEd, APIs for working with OpenXML using .NET. The APIs are built on top of the System.IO.Packaging APIs (.NET 3.0). A couple of open source projects (CodePlex and elsewhere) started on this, but good to see something coming out of Microsoft. (could be the same project, since I think the one at CodePlex was by an MS employee)
http://blogs.msdn.com/usisvde/archive/2007/06/04/microsoftsdk-for-open-xml-formats.aspx