iOS - How to draw a YUV image using openGL

Currently, I am trying to draw an image using openGL (the image updates very often, and thus must be redrawn). Previously, I was converting my image from YUV to RGB, and then using this new image to draw with openGL. All worked fine, but the conversion process was not particularly fast.

I am now attempting to change the code so that the conversion is taken care of in the openGL shaders. After looking around, I've found a couple code snippets (particularly the shaders and the bulk of my renderImage function) that have helped me get a baseline, but I can't seem to actually get the image to draw properly - all I ever see is a black image.

It's quite likely that I'm missing something extremely simple and important - My experience with openGL is rather limited. If anyone can take a look and see if they recognize anything wrong, please let me know.

I should point out, I'm trying to support iOS 4.x, so CVOpenGLESTextureCacheCreateTextureFromImage shouldn't be usable (and I'm not really positive how to setup to use it even if I wanted to).

Any help would be appreciated. Code below -

My Vertex shader:

attribute vec4 position;
attribute vec4 inputTextureCoordinate;

varying vec2 textureCoordinate;

void main()
{
    gl_Position = position;
    textureCoordinate = inputTextureCoordinate.xy;
}

Fragment Shader:

#ifdef GL_ES
precision mediump float;
#endif

varying vec2 textureCoordinate;

uniform sampler2D videoFrame;
uniform sampler2D videoFrameUV;

const mat3 yuv2rgb = mat3(
                          1, 0, 1.2802,
                          1, -0.214821, -0.380589,
                          1, 2.127982, 0
                          );

void main() {
    vec3 yuv = vec3(
                    1.1643 * (texture2D(videoFrame, textureCoordinate).r - 0.0625),
                    texture2D(videoFrameUV, textureCoordinate).r - 0.5,
                    texture2D(videoFrameUV, textureCoordin