If you're going to be at TechEd in Boston next week, be sure to check out Brian Jones's presentation on the Office Open XML file formats. It's Wednesday at 5:30, and Brian has all the details on his blog.
If you've been doing any development around Open XML since the release of Beta 2, head on over to OpenXmlDeveloper.org and share what you know, or get your questions answered. We're starting to see more activity now that Beta 2 is out and the working draft of the Ecma spec has been published.
A good example is Wouter van Vugt's handy Package Explorer utility, which was posted on Codeplex today as a shared-source community development project. Other Open XML projects will soon appear there as well, so now's a great time to start ramping up on Open XML development.