The models/depictions I've seen of warp bubbles show space compressed ahead of the bubble and expanded behind, so that the space inside the bubble moves with respect to the space outside. If that is so, then what is happening at the sides? It would seem that there is some sort of shear taking place between the space inside and outside. Is that correct, or is that based on a misinterpretation of the model?
See also: http://arxiv.org/abs/gr-qc/0009013
the outside of the bubble is causally disconnected from what happens inside the bubble. So that means that the space around the bubble doesn't even know there is a bubble. Even if you had the exotic matter to do the trick, you would have to lay it out as a cylinder between home and destination before you can make the trip. So, as an interstellar propulsion, is without a doubt useless
this is why the Alcubierre drive is a no-go; compared to this problem, the lack of exotic matter (aka negative mass) is just a minor detail
If you had the exotic matter, you could probably try to do this on a lab, arranging the field beforehand. But of course, you don't have exotic matter either