A crucial part of the Gregor system is our binary XML format, called XDM, for "XML Data Model." Naturally, it is more compact than serialized XML which makes it a good option for storage and transmission of XML documents.
But what is really important about XDM is that it is preparsed and that XSLT/Xpath logic of translets can be applied to its XML data in the nick of time.
In our eCarrel project, we use XDM to store EPUB book chapters in Google App Engine's (GAE) DataStore (EpubDB). We also preprocess EPUB's XHTML into XDM for use w/i the Android (Honeycomb) eCarrel client.
NEW: Combining MongoDB, XDM, and translets leads to a scalable, fast, highly functional GregorXmlDB.