Fetching cubemap alpha channel (using GLSL)

Hello,

I recently gave a try at cubemaps on my iTouch (3rd generat) using my Unity 3 Pro licence. I can successfully create a cubemap unity object and assign 6 different images to it. I can also sample it within the shader (GLSL for OGL ES 2.0).

However, once I sample the cubemap alpha (I'm not actually making a different sample, just reading the .a component of the fetched texture) it only outputs white, which is the default value of my sampler. edit: I forgot to mention I'm using 32bits tga files for the 6 faces of the cube, which so far hasn't caused any trouble concerning regular textures' alpha.

Is this a bug? Are cubemap only made to store RGB information and not RGBA?

This would be very useful if not essential for anyone looking into more advanced lighting tricks, especially since this is all very standard behavior.

Thanks