I have been toying with the idea of building a bunch of decks based on the following cards for ISD+RTR Standard:
4 Drainpipe Vermin
4 Typhoid Rats
4 Ravenous Rats
4 Pack Rat
From this rat shell I can go in many directions, but I am not sure which color combinations will be the best.
White/Black (Orzhov) will probably have to wait for Gatecrash
mechanics. Also, exalted does not go well with rats because what you
want is to generate a swarm of rats, not to hit with individual rats.
3. Blue/Black is another Gatecrash wait deck. Also, the rats let you run fast early. I don't see the need to slow the game down.
4. Rakdos Rats seems to be the best option, especially an aggro build.
5. Golgari Rats also seems like a good option because with green I can boost mana, therefore boost generating rats. There is also Scavenge as an ability.
Here is an online Golgari Rats deck.
November 4, 2012 update: Here is a monoblack rats deck that actually survives many rounds in tournament practice!!