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Is line of sight described somewhere in the rules? It references it once when talking about targeting, but I can't find a definition....

visible to the attacker (if
unsure, stoop down and get a look from
behind the attacking model to see if the
target is visible). For the purposes of
determining visibility, an attacking model
can see through other models in its unit.

Yep, so you can shoot through enemy units without penalty as long as you can see the target in some way.