Is it one large 2d texture and then you split it into 6 parts and render depth to each one of them? It sounds strictly better than cubemapping, is there any cons to it?"
I'm assuming what hes doing is still rendering the scene 6 times and 6 views, just changing the glViewport to write to a different portion of the texture.
What you (Johan) have suggested can't be done, if you are implying that you render the scene once and just have it all map to several parts of a texture. The problem you have is if you have 6 squares randomly packed into a single 2D texture, when you generate the vertex coordinates, any given triangle if only rendered once, can obviously span multiple "squares". There is no way to get one part of the triangle to go to square1 and another part to go to square 2. If you could write in the pixel shader the exact pixel location you want a fragment to end up at, then it would work. But the pixel shader runs after a triangle has been made in screen space and chosen which pixels locations need to be run on the pixel shader.
My question to the original post is, whether you render 6 times to a cube map or render 6 times to a single texture acting as a cube-map, why not just use the cubemap. Not to mention that cube maps are hardware supported and when you want to sample on the edge of two of the images, it will sample them properly. In order to do bi-linear filtering across your "faces", you would have to waste pixel shader time to resample the texture instead of letting the hardware do it.
Of course there is dual parabaloid maps.