Well, I'm not sure how I never ran across it before, but Philip Rideout, author of O'Reilly's iPhone 3D (a book I recommended last week), has a hell of a blog for people interested in OpenGL. A lot of the posts are specific to desktop OpenGL (and recent versions of OpenGL at that!), but there's tons of great information that applies to OpenGL ES as well. It's a veritable treasure trove of advanced graphics goodness from a guy who's been doing it for years.