It's a WordPress theme that Google have sponsored to allow authors to migrate their Google Knol articles.
For everyone else, it's a new open access scholarly journal publishing system built on WordPress (hosted on wordpress.com and available for download). I've not looked at the code yet, but on the face of it, I wonder whether digressit might benefit from some of the workflow features they've introduced, as well as the work on citations and import/export.
A merge of digressit and annotum would be a dream for many of us working in higher education.
Joss