honestly I have no idea why i'm writing this.
probably because i know i will forget and well, if i write stuff down, i probably have a better chance of remember it.
this is a pretty simple problem, but oh well ;)
i once forget how to calculate a cross-product :|
 
 

basically to get the answer to see if a ray/plane have intersected you break the problem up into 3 little chunks.

a.) obtain the equation of the plane of that polygon.
b.) now, plug up the ray data into the plane, and see if it intersects.
c.) check if that intersection is in the "actual polygon".

well, that's all ;-)
i bet i'm the only that ever going to come back to this page ;)

peace,
akbar A.