From the “Ask the Kobold” thread http://www.koboldquarterly.com/kobold_kqforums/viewtopic.php?p=4509#4509

Me: 2a. Does the viper threaten?
Skip: Yes.

Me: 2a(1) If yes, why does a swarm of Tiny creatures not threaten?
Skip: Because swarms don’t threaten squares, see the swarm subtype description.

Me: 2a(2) If yes, would 100 Fine creatures in the same square each receive an AOO as well?
Skip: See 2a

Me: 2a(3) If yes, since Tiny, Diminutive, and Fine creatures can each exist in the square at the same time, does that mean 4 AOOs from the Tiny, 25 AOOs from the Diminutive, and 100 from the Fine?
Skip: Frankly, I’d force critters that small into a swam, just as a practical matter.

So would you agree then that four Tiny creatures in the same space would all threaten the Medium-size creature — there’s no way that four Tiny constitute a swarm, right? Otherwise, four sprites or four black dragon wyrmlings constitute a swarm?!

And since 25 Fine can fit in the same square, all of them would threaten as well? And I understand that you would lump the 100 Diminutive creatures into a swarm, but given the way you’ve ruled in this article, wouldn’t it be within the spirit of the rules to allow those 100 Diminutive creatures their AOOs?

And would I be within the rules to simply make statistical assumptions about the AOO attack rolls? For example, say there are 20 Fine creatures. Can I assume one of each for a 20-sided die?

I have to ask because I fully expect my players to follow this same line of reasoning and I need to be prepared!