When I read the title, I thought the "Mystery Meat" was going to be like the "Strange Meat" from Fallout 3.

I thought so too, but since it's just filling, I'm both glad and dissappointed. Still, just saying it's mostly filling isn't very helpful, since we don't know what's in the filling. Even so, I'm glad to see that someone is taking action against this, even though it's not at all a shock that the supposedly 100% pure meat isn't actually that at all. When I get fast good, I'm certainly not expecting to get healthy food, but I'd rather not be decieved as to what I'm actually eating. Either deliver what you promise, or stop making false claims. But with this sort of thing, to change it, someone needs to show them that this is not the kind of treatment their customers want.
Granted, I don't think I've ever once eaten at a Taco Bell anyway, but I'm sure that lots of other fast food places are doing similar things, it's just a question of which ones we know about.