Unarmed strike is their natural attack simply because without any other weapon, that is what they would attack with. The reason it doesn't necessarily fit the standard definition is because of the special modifiers on many humanoid races that make their attacks nonlethal. That, however, doesn't make them any less natural. It's an attack using a part of your body, that simple. There is nothing explicitly stating whether or not it's right, but the spirit of the rules is clear on that point.
It seems, however, that people around here (namely the previous two posters before Dimwhit) don't like to read entire posts. You see, I made two points. The first was merely about natural attacks in general, the second was about the rule itself. You attack the first part even though I didn't use that in the second part because it's truly a moot point. Anyway, since you didn't pay attention the first time, I'll repeat myself word-for-word. I suggest reading the entire post this time so you don't make yourself look bad again.
So yeah, READ THE WHOLE THING! In fact, I'll even underline the parts you conveniently skipped so you don't miss it again.
I had a three-day vacation? Didn't even notice, I was playing RTKX the last few days without being on the internet. So much for that, heh.
Anyway, I saw someone mention on the other thread that no one considers a human monk's unarmed strike a natural weapon. I'd really like to know how this could be possible. You see, we have all this stating otherwise:
A human's natural attack is it's unarmed strike. A monk's unarmed strike ability makes that natural attack lethal with more damage, and is thus a special ability modifying an existing natural attack. Improved Natural Attack would be applied next, but on top of the existing abilities, not parallel to them. As such, the monk's unarmed strike is an ability-modified natural attack, and perfectly qualified for the feat. In fact, I'd go a step further and say that any human could take it, not just monks, but the effects would be pretty dang weak otherwise.
Anyway, I still don't see the debate here, not for the Rules forum. This forum is about the official rules. As-written, monks qualify. As per the Sage's ruling (and he is the official rules interpreter), monks qualify. According to Artoomis (I think it was Artoomis), customer service says monks qualify. The spirit of the rules suggest monks qualify. Based on all this with a dose of common sense and the word of people "in the know", the official stance is that monks qualify, and that means specifically that per the rules, monks qualify.
Now whether it's balanced or not, that is well and fine for a debate. Whether you wanna house rule or not is up to you as well. This is the Rules forum, though, and the official by-the-book and by-the-creators (Skip Williams's name is listed as being on the design team, so he is most certainly one of the creators) rule, for purposes of a Rules forum, is that monk's qualify.
Honestly, what gives anyone here the right to defy the very creators of the product on this? They made the rules, so what they say goes, at least officially speaking. Use whatever you want at your table, but if you argue it any other way, it's not the official rule anymore and belongs in another forum. The case is closed, and from the Sage's mouth to all your ears, one truth prevails.
Heh, and without a single insult. I must be happy about my overwhelming victory over Sun Quan last night.