Firstly, there was a typo error in the script I mentioned. It should have read as
sky skycube
{
type = <skycube5+6.tga>; // Note: filename extension was wrong
flags = cube;
layer = 2;
}

Secondly, all I can figure from all the browsing I have done in these forums is that the terrain face of the skycube was never meant to be walked on. I can bypass the problem by using the skycube in conjunction with a *.hmp terrain.

Someone please correct me if I'm wrong. Thanx.