Not an official API but this guy seems to have reversed engineered some of it.
http://notanothercodeblog.blogspot.com/2011/02/google-goggles-api.html
You may also want to try VisualSearchAPI.com, they have visual search APIs. See demo and docs here. http://www.visualsearchapi.com/demo/view?demo=1 and http://www.visualsearchapi.com/apiDoc/apiDoc?apiDoc=V