It somehow doesn't work properly.
1. If I have the material assigned to more than one entity all entities use the skin of the entity which was last assigned with the material.
2. I don't have a "ceiling".
My cube map is a 3072x512 bitmap (512x512) named cube+6.bmp. First four parts are the sides, than the sky and finally the floor. Ingame there's no sky to see, only a black square.

I'm using a GeForce2MX400. Not the best card but capable of DX7.