I recently discovered that my own site was being filtered by a system in use at a friend's employer (he works for a large financial organisation in the city). I can't recall the name of the product, but it's apparently one also in use by a number of totalitarian regimes around the world.
My site was apparently categorised as being about 'games', and so was blocked (I've not blogged about games in over a year, and no articles on that subject exist on it since I deleted the site by accident back in October). I would guess that this can't be the same system as you're experiencing, but it's all part of the same ridiculous premise.
