Pete posts an interesting picture of the new Merrell Barefoot Trail Glove:
In this side view, you can see pretty clearly that even though this is a zero-drop shoe (it does not have a raised heel), it was built using a last that had a heel.
When I had my New Balance MT100s dropped, I had the opportunity to learn a lot about shoe-making from the cordwainer. (I probably learned very little about shoe-making, actually, but it was a lot compared to what I knew at that point.)
One of the things I got to do was see a whole bunch of lasts, of various vintages. A last is the model of a foot that a shoe is made around. A last can be a mold of a particular foot, but more often it's sort of an idealized foot. Since a shoe is made of materials that aren't perfectly flexible, certain adjustments are made to the last to accomodate the movement of the foot when it's in the shoe. Toe spring is the curve up under the toes, to accomdate the movement of the foot while walking or running.
Another adjustment is that the last is designed as if the foot were bent in the way it would be if it were standing on a heel. (Look at the curve of your foot when it's unweighted to get an idea of the shape.) Very few lasts are made without heels (the cordwainer had some, in fact), and lasts are expensive to make, about $600 per last. And yes, you need a last for each size shoe you're going to make.
Russell did when they made my shoes: they just used a last with a heel built in, even though there was no build-up under the heel in the actual shoe. If the material of the shoe is flexible enough, there's no reason why this should be a problem. My Russells now look completely flat; you can't tell that the last originally had a heel built in, but leather is more adaptable than the plastics the Merrells are made from.
This does help keep the price of the shoe down, which is a good thing. Barefoot-style shoes, being a niche market, have tended toward the expensive side.
Oh, and if you're wondering, I'm wearing a pair of Russells today. I wear them almost every day. They're hands-down the most comfortable minimalist shoes I've found, and they're the most comfortable shoe period I've ever worn. Yes, they're on the expensive side, but sometimes you really do get what you pay for.