I need to support the fast rendering of huge quads in the order of 10,000 x 10,000 pixels.
Either in general or specifically to the iPhone, does OpenGLES clip texture drawing to the current viewport automatically? Or do I need to add some code to trim these vertices down to the size of the screen?
I've seen talk about optimizing for a lot of vertices, but what about only 4 vertices in a very large textured quad?
The OpenGL rendering pipeline performs clipping and culling before rasterisation — so there's no per-pixel cost for parts of geometry outside of the viewport.
If you know that your geometry will always exactly fill the viewport then you have more information than you've disclosed to OpenGL and could in theory write code to get to your output geometry in fewer operations. In your case, you'd want to work backwards to project into the world and find the four points that go at screen edges, probably in a vertex shader. However the difference should be so negligible, even if you wrote an absolutely optimal solution, as for it not to be worth the extra code burden.