Beta 2 of the 2007 Microsoft Office System was released on May 23. This is a public beta, meaning anyone can download a copy. Now any developer can get their hands on software that generates and consumes Office Open XML documents.
You can sign up to download the beta at the 2007 Microsoft Office system preview site. If you're writing code that uses the WinFX packaging API to read and write Office Open XML documents, you'll also want to download the WinFX February CTP. For more developer info regarding Beta 2, Jensen Harris has posted a great list of Some of the Best Beta 2 Resources.