The new XML file formats in Office 2007 open the door to lots of exciting programmability solutions. For example, the files can be created and consumed without automating Excel.exe. This really helps server-side solutions. It also aids batch processing of Excel files and cross-platform business solutions that exchange Office files.
To enable easy access to the files, we provided the .NET Packaging API. But this is a low-level API which does not have any application awareness. For example, it doesn't understand Excel ranges or tables; it deals with the raw XML elements and relations.
But we are working on improving this situation.
The Open XML SDK is a .NET library which will provide a higher-level API for the Open XML files. It will have application awareness and will greatly benefit developer productivity.
Version 1.0 will be released in May 2008. This will be an augmentation of last year's CTP. However, there won't really be anything in v1.0 for Excel developers. The Excel support will come later this year.
Here are some links which contain more information on the SDK: