Further to the alternative storage talk on Tuesday, http://trac.openmicroscopy.org.uk/omero/wiki/AlternativeStorage. We have created a new page http://trac.openmicroscopy.org.uk/omero/wiki/WorkPlan/AlternativeStorageMongoDB which is looking at the use cases that MongoDB should satisfy.
If anyone has suggestions could you reply to this thread and it'll be put onto the page.