Eric's blogged about a new open source initiative on CodePlex for processing Open XML documents using PowerShell. Very interesting. He's even made a video showing how to get up and running with it quickly.
The PowerTools include a host of PowerShell cmdlets for performing various actions on Open XML documents which can be composed into PowerShell scripts. It also includes sample scripts as well as guidance on building additional cmdlets to manipulate Open XML docs. PowerTools for Open XML is built using the new Open XML SDK.