Currently, E:LH has a 'weighting' system for where it places units. Brad and the guys have assigned parameters that choose where a unit is placed, based on what equipment it is carrying and such. We currently do not have the ability to re-assign the order of units ingame.
Essentially, melee units in the front, missile weapon users farther to the back is how the algorithm generally works.
It has been suggested by myself and others that a means to shuffle unit order, and perhaps even assign formations to units, is something many of us would like to see. I'm guessing that if we ever do see something like this, it'll be in an expansion.
Brad has specifically stated that he does not want a 'pre-placement' system in Elemental, where you choose which squares to put your units in on the map before battle commences. But a more general 'formation' option, which attempts to place as best units as best it can, based on wherever the game decides to place your formation - think fog of war -, and shifting individual units away from prohibited terrain such as fences, trees, etc., hence disrupting the formation, well this might be something Brad could live with.