I would like to share a site that allows free streaming of documentary films on the iPad. it is basically an aggregation and curation service. There is no app, but the web-version should be fine as there are no flash videos once the site is loaded from iOS. this is one major difference to those many other free documentary sites and second, all films are legal uploads (from filmmakers themselves)
Documentary.net - Watch the best free documentaries online
hope you find this info useful.
(we are some doc-fans who missed a nice and worldwide available solution for the ipad)