Went there for the first time last year for the GP, found a few nice spots.
1) Pathway alongside Eau Rouge: There's a spot where you can shoot through the trees looking up the corner, Great for some naturey shots.
2) The left hander after Rivage. You shoot looking down right on top of the cars, its a brilliant spot and I got some great pics of the GP2 cars there.
3) Entry to Rivage: Wierdly there's some huge metal wheely bins (doesn't look like they move) that allow you to get shots over the fencing as the cars exit Malmedy and head down the hill. Bit sneaky but worked well!
4) Malmedy. On the inside of the circuit theres a little path in the trees that runs right by the fencing on the inside of the corner. Quite tough to get pics as you will spend most the time trying to keep your balance but definitely worth trying. Access is via a small pathway to the left of the pedestrian tunnel under the straight between Malmedy and Rivage.
5) Bus Stop: the new layout means you can get great shots as the cars brake into the corner and turn right. If you are heading away from the pit straight, walk past the grandstand below you until you reach a (probably closed) waffle hut. From there there ought to be a section with camera holes in the fencing. You will get awesome shots from there.
6) Blanchimont: The public pathway runs slightly above and close to the track here. If you can climb up the hill a little it and shoot down over the fencing you should get great shots of the cars through the trees. Loos great if your camera is up to the task!
Hope this helps