I touched on this in my blog, but I think the blame lies primarily with the wearer - though the Vibram shoes allow the injury to happen. That was probably gibberish, so let me explain.
Most people in the Western world are used to walking and running in relatively rigid and padded footwear. The combination of a raised, cushioned heel and a lack of flexibility in the sole of the show results in a gait that favours a heel-strike.
To see just how dramatic this can be, I recommend that everyone try the following experiment. Put on your favourite shoes and walk a few steps as naturally as you can at your normal walking pace. If you pay attention, you'll see that when you step forwards, your heel hits the ground first with a relatively strong impact, and your knee will be fairly straight.
Now take off your shoes (and socks) and walk again. You should find that you walk quite differently. You will place your feet down relatively flat with each step, and most of the weight will be concentrated along the outside edge of the middle section of your foot, before the foot flexes upwards as it prepares for the next step. There is no harsh heel strike, and your knees will be more flexed at the point of contact with the floor.
You could try the same experiment with running and note the differences. You may try to run with a heel-strike when barefoot, but the nerves in the soles of your feet very quickly tell you that something is wrong, and your stride will naturally adjust. Without cushioning, the stress of the heel-strike becomes painfully obvious, as it sends a huge shock up through your heel, ankle, knee and hip.
The key is that the soles of your feet are extremely sensitive to the floor surface and allow you to make micro adjustments to the way that you step and land, and one of these adjustments is the natural avoidance of a heel strike, or of heavy landings from over-extended strides etc.
Now consider what minimalist shoes do. They remove all the support and padding, but isolate the soles of your feet from the ground, starving them of sensory feedback. As a result, it's just possible to run with a heel strike and long strides, because your feet feel some degree of protection, although in fact there is very little, and certainly no cushioning. Therefore, I can see how it would be entirely possible to suffer stress fractures by running in minimalist footwear if one is not already familiar with barefoot locomotion.
As a result, I would strongly advise anyone considering running in minimalist footwear to first walk and run fully barefoot for a while in order to learn how to do it, so that technique can be applied when running in minimalist footwear.
It's important to take the transition slowly, as it takes a fair amount of adjustment for a person normally used to running in modern running shoes. If you run in the same way while wearing minimalist shoes as you do when wearing modern running shoes, I think injury is inevitable.