I was just having a look at the Office Developer Center on MSDN and I noticed this blog post from Erika Ehrli on the new Open XML Object Model.

The Open XML object model simplifies the code you need to write to interact with Open XML documents and goes beyond the document manipulation contained in the System.IO.Packaging APIs. There is a Community Technical Preview available for download so you can start working with the new API and it includes an SDK as well (or the SDK documentation is available online here).

Brian Jones has posted a couple of simple samples on his blog that show how easy it is to delete a part in an Excel spreadsheet and how to retrieve the comments from a wordprocessing document.