Robert_Charlton - 7:09 am on May 10, 2011 (gmt 0)
Currently, "Sort by Subject" appears to be just at beginning stages... not yet a wide range of subjects. Some of the subject areas, though, have subcategories.
Playing with it a bit, I find it's helpful to combine this feature with the "Similar" images feature once you see something close to what you like. Use the "Similar" links that appear in the overlays when you mouse over images.
If you start with text search in Images, you can sometimes further "Sort by Subject", and then increasingly refine by "Similar" choices. Note, though, that in "Similar" results, color palette and composition also become strong factors, sometimes overriding Subject.