I was sure we cooked up some good swarm rules last year, but all I can find is this: http://www.brikwars.com/forums/viewtopic.php?p=5516
BW2010 is getting new rules for Vermin units. Like the Cannon Fodder units, Vermin have Damage 1 and Armor 0 rather than die rolls - any hit will kill them, so swarming will be essential to survive and stack enough damage to do any good.
My current thinking is that any regular attack will kill them only one at a time, but area attacks like flamethrowers and explosions will be an almost guaranteed TPK. Because of their tiny size, most units won't get Counters against them in Close Combat, but they can make attacks against the swarm size rather than the individual Vermin size for determining attack modifiers.
In my playtests I've been putting them in squads with minifig units to serve as cheap damage sponges and to pile on skill penalties from multiple CC attacks, while the minifigs hide behind them and dish out full weapon damage.
I might add some kind of balancing factor in later, for instance to say that once a target is getting swarmed, any successful attack against either the target or the swarm automatically hits them both. (Naturally the swarm is exempt from hitting itself though.) This'll make it a little more costly to have combined squads of Vermin and regular minifigs.