Enabling alpha channel in OpenGL ES

How can I modify the alpha channel size through the CAEAGLLayer class (or whatever else I could do it with) used to set up the rendering for a opengl es iphone project?

Another way to put it; how do I accomplish what this CGL code does to set pixel format attributes (CGLChoosePixelFormat)?

Using CGLChoosePixelFormat() you can pass in properties such as kCGLPFASampleAlpha and kCGLPFAAlphaSize. How do I get to specify these starting from a standard opengl es iphone project (which I'm not sure if it can use CGL functions).

If it helps I have an old opengl GLUT project that called:
GLUTInitDisplay(GLUT_DOUBLE | .... | GLUT_ALPHA)

or it might have been GLUTInitDisplayMode... not sure.

Where the GLUT_ALPHA accomplished what I needed...

Sorry for the long post, any help is very much appreciated!