yeah, i had already tried that, from the tutorial on www.gametutorials.com, it shows you how to get the cameras frustum by multiplying the modelview and projection matrices together then extracting the sides, my problem is is that everytime i tried dotting the normal of the bottom plane and the normals it screwed up.
About the data structures, ive already got a quadtree structure splitting up all the terrain data and everything, so ive already culled out like a shit load of glVertex3f calls so i was just curious if backface culling would make it any faster by removing even more. It doesnt necessarily cut out large chunks really, only 4 vertices, but i thoguht i might aswell try it and see what happens, the only problem i have is that when i test if the poly''s are front or back facing, it comes up with some really weird results, anyone know of an OpenGL implementation that does this??