I agree that they are here to stay in one form or another. They may end up being more regional due to over regulation in some areas. The point made by others regarding the lower overhead compared to a B&M is very valid and imho this is what will keep them around.
A B&M has some advantages but a food truck has advantages over a B&M. With a truck you can "serve" multiple markets. We have friends in the BBQ biz that have one and they use Twitter and FB to inform their customers where they will be on any given day. In rural areas like ours the ability to "move" your business around increases your sales and ups your gross which would be near impossible out of a B&M that is anchored in one small town.
Personally, I hope they do stick around. The B&M's need some healthy competition.