As James Senior beat me to saying, Microsoft has created an SDK for .net programmers to manipulate Open XML documents. The point is that whilst it is technically possible to write a .docx or an .xlsx using the Ecma spec, it requires a detailed knowledge of the tags. You'd need to be able to unpack a .zip, parse say content.xml, alter or add tags, resave and repackage as a .zip. The SDK takes all the hard work out of it for you and includes handy samples for doing the most common tasks. This will really help with the development of document assembly and workflow solutions. Partners are starting to talk to me about the work they are doing with their customers now (I hope to put some interviews up at some point). If you have a good story in the UK for me please get in touch.
There is rather rambling channel 9 interview with Chris Bryant from the Office Business Applications group who built it if you want to learn more. Although the whole darth vader thing disturbs me.
Download the sdk here.