I was having a look at the army list and wondering whether a 'jungle ambush' themed army - lots of spider riders, an arachnarok etc, with lots of movement (and the ability to ignore terrain) instead of the usual horde approach to the army - could work.

Spider riders are expensive in that they cost roughly 4 goblins, but their ability to move pretty much wherever they like and the poisoned attacks of the spiders make them look very effective in an army built for speed and flanking the enemy. They're also cheap enough to take the place of conventional scouts.

So. Has this been tried (sorry if there's a whole thread on it, but I had a quick look and couldn't find one)? If it has, has it worked?

The only drawback I can see to this is a lack of numbers. You're looking a single "unit" for Spider Riders as opposed to four "units" for the same cost. Could make a big difference when the dung starts getting flung.

It is a good idea though, especially if the table is full of terrain. The Spiders could use that to their advantage and stay out of lines of fire, and then simply use their full movement to cut through the terrain and get at enemy regiments.

L4 Goblin Shaman on Arachnarok
Characters (one to babysit Trolls mounted on a gigantic spider...I like to get a BSB with the Ranger Standard)
Snagla Grobspit (gives unit ambush)
~50 Forest Goblin Spider Riders with spears (50 ambushing forest and wall strider horde with devastating charge)
1-2 more Arachnarok spiders
Fill in the rest with Trolls.

Make sure that the Forest Goblin units are big enough to not only be devastating on the charge, but can take a round or so of combat. pretty much every army in the game now fields units of 30-40 people, so make sure that you can either charge with both a unit of Spider Riders, and an Arachnarok, or charge the front and do a flank charge with some Spider riders at the same time.