It looks like we've all eaten it; horsemeat that is. It has been found in beef, lamb and chicken and the major supermarkets are still not giving categorical assurances that it is out of the food chain. I haven't heard of it being found in pork pies yet but that can only be a matter of time. (Pork pie eaters won't be upset - they know the deal when you eat a pork pie. It's like gays in the US military: don't ask, don't tell.)
It may be slightly disconcerting to be eating horse when you wanted to eat cow but if truth be told it is probably a better meat anyway, less fat, especially if the horse in question was Romanian and was worked until it dropped before being tossed in the vat. It is unlikely they gave it any drugs we need worry about, unlike cows which are stuffed with hormones and antibiotics and fed on meal made from each others brains (OK that was a few years ago, but UK cows are still heavily medicated.)
No, the real problem is that there is a backdoor into the food supply chain. And anything could get in through that backdoor. All animals entering through the front door of a slaughterhouse are checked by a vet - they must at a minimum be able to walk on their own four feet and there are a battery of other tests. And they have to show their passport to get in. (If they've lost their passport they are unfit for human consumption and their only option is to become chickenfeed. The resultant eggs are fit for human consumption.)
Anything could enter through that backdoor. Infestation of rats in the slaughterhouse? Don't waste them, sling them in the vat. Stray cats and dogs? All grist to the mill. The output of that vat is known technically as "pink slime" and is an essential ingredient in many foodstuffs, especially convenience food such as microwave meals.
Did you notice how the attitude of journalists reporting this story changed suddenly when Waitrose admitted their meatballs tested positive for horse? Previously they had been fairly nonchalantly reporting problems at Aldi, Tesco and the likes - then suddenly Waitrose is on the radar and it has become personal.
The nationalist solution is to ban imported processed meat; livestock fine; whole caresses fine; sides of beef and other recognisable cuts are acceptable, but minced meat and meat sludge should be turned back at the border. We can police our own meat packing industry; we cannot police the world.