Hi,
If I have 3 vertical parallel edges and select the top vertex on the left edge and the bottom vertex on the right edge and press F, I get the expected new edge connecting the vertices and crossing over (overlapping) the middle edge but no new vertice is generated for the middle edge at the crossing point. Is it possible to merge the new edge with the underlying mesh, in this case the middle edge and form a vertice at the crossing point? I know I can use the knife tool with exact cut to get the desired result, but it is not as accurate and usually generates new vertices for the left and right edges that have to be merged with the original vertices which is a lot of work! 
You have to create 2 faces instead. In edge mode select the right and middle edge and press f then select the middle and left edge and press f.
Edit sorry I misread your post – search for an intersection script – it will do what you want
link http://www.fourmadmen.com/blender/scripts/4mm_edge_intersection.html
GreyBeard