The spell Insect Plague summons locust swarms composed of 1,500 locusts per 10 by 10 feet square (shapable). This should limit sight severely. But is there any rule that it does?
The Rules Compendium has a "few lines" about that...
It says under the Line of Sight section (emphasis added):
Line of Sight Line of sight establishes whether you can see something else represented on the battle grid. Presupposing you can see, determine line of sight by drawing an imaginary line between your space and the target’s space. If any such line isn’t blocked, then you have line of sight to the target, and if it’s a creature, it has line of sight to you. The line isn’t blocked if it doesn’t intersect or even touch squares that block line of sight. If you have line of sight on a target, you can see to cast a targeted spell on it, shoot it with a bow, and so forth. If you can see a portion of a large creature that occupies more than one square, you can target that creature for a spell or any other attack.
If you can’t see the target, such as due to being blinded or the target’s invisibility, you can’t have line of sight to it even if you could draw an unblocked line between your space and the target’s.
If line of sight to a target is completely blocked, you can’t cast spells or use other abilities that require line of sight to the target. When line of sight is blocked by something that doesn’t otherwise physically block or prevent an attack that doesn’t require line of sight (such as fog), you can still make that attack, but your target is treated as if it were invisible (see page 76). If line of sight is partially blocked, such as by the corner of a building, attacks work normally, but the target’s AC increases due to the cover.
Two creatures can see each other if they can trace at least one clear straight-line from any part of one creature's space to any part of the other creature's space. The line is clear if it doesn't cross or even touch squares that block line of sight.
Now, the tricky part is that I'm not able to find anywhere the explicitly says that a creature does, in fact, count as blocking line of sight. It does say that "solid barriers" block both line of sight and line of effect, with a caveat that if there is a large enough hole that the barrier no longer counts; however, a creature is not usually considered to solidly take up the full area represented by the spaces it is listed as taking up on a battle grid; even, or perhaps especially, in the case of a swarm. This is why it is possible to pass through an occupied space (but not end one's move therein) under specific conditions.
In my opinion, having watched swarms of bugs, flocks of birds, and mischief of rats, a swarm would provide partial concealment but not otherwise block line of sight, as one can usually catch brief glimpses of the other side through the shifting mass of creatures. Here is a video supporting my opinion.
Since this is "D&D Magical Physics", however, A DM could feel free to rule that a swarm is more (or less) 'dense' and therefore does (or does not) block line of sight.