My gut feeling is that wildshaping into a swarm is not RAI, but I've had such feelings at times and been wrong, so it feels worth asking.
When it comes to RAW, so far I cannot find anything that says you could not.
At 4th level, a druid gains the ability to turn herself into any Small or Medium animal and back again once per day. Her options for new forms include all creatures with the animal type.
Wild Shape includes any and all creatures of the Animal type, and while swarms are their own subtype, they are a subtype of Animal, which should be inclusive of the "parent type", right? Or is my programmer's thinking messing this up?
One could say that a swarm is many creatures, but...
Swarm Subtype: A swarm is a collection of Fine, Diminutive, or Tiny creatures that acts as a single creature.
The best I can come up with to counter this is that it says "acts as" and not "is a".
Finally, there's the issue of size for lower level druids, but my question is not restricted to early levels. So, if the druid can turn into fine or tiny creatures, are swarms okay?