I'm trying to settle a coffee break argument - does a weapon proficiency for unarmed strike exist (either explicitly, or under some other more general weapon proficiency) under D&D 3.5 rules, and, if so, are monks automatically proficient in unarmed strike?
One: The Unarmed Strike is a Simple Weapon
It’s listed in the table of simple weapons. The errata rules clarify that in the case of a contradiction between rules text and a table, the text should be taken as correct (“text trumps table”), but no text states that Unarmed Strikes are anything but a Simple Weapon. Ergo, that is what they are, despite being very strange weapons in general.
Two: The Unarmed Strike is also sort-of a Natural Weapon
Despite being a simple weapon (a classification of manufactured weapon), and despite the fact that it generally follows the rules of manufactured weapons (e.g. iteratives), the Unarmed Strike is frequently treated as a natural weapon. You can use magic fang but not magic weapon on it (unless you’re a Monk), you can take Improved Natural Attack (unarmed strike), and so on.
Three: Humanoids do not automatically get proficiency in their natural weapons
Unlike most creature Types, the Humanoid type does not grant proficiency in its own natural weapons. Compare Humanoid to Animal or Dragon or what have you.
For the record, Fey, Monstrous Humanoid, and Outsider types don’t grant such proficiencies either. However, they all get proficiency in all simple weapons, which leads me to...
Four: The Humanoid type does not grant simple weapon proficiency if you have class levels
The Humanoid type, unlike the other types that lack automatic natural-weapon proficiency, does not get simple weapon proficiency if the creature has class levels.
Five: The Monk class does not grant simple weapon proficiency, or proficiency in unarmed strikes
Neither is listed, and Monks are explicitly one of the few classes that doesn’t get the entire simple weapon proficiency.
Six: Improved Unarmed Strike neither requires nor grants proficiency in unarmed strikes
Nowhere in the feat’s text is proficiency mentioned. The Monk’s better version of the feat does not either. Both deal with unarmed strike damage rolls; proficiency is about attack rolls. There is no inherent contradiction between a Monk taking a penalty to attack rolls (from nonproficiency) and getting a bonus on damage rolls (from the unarmed strike feature); after all, that is how Power Attack is supposed to work. It’s just a matter of it not making much sense for Monks to lack basic proficiency with the weapon they are supposedly masters of.
A Humanoid Monk does not have any feature granting him proficiency in unarmed strikes. As a result, the default case holds true: he’s not proficient.
This is universally agreed to be an oversight. The Monk is a very-weak class; it really does not need to either lose its best class feature or take a −4 penalty for using it. Just proof that Wizards did not have a particularly impressive editing staff for the D&D 3.x project.
Humanoid Monks are not proficient with their Unarmed Strikes.
This is because proficiency with Unarmed Strikes, the ability to use Unarmed Strikes to deal lethal damage and be considered armed while using them, and the Unarmed Strike class feature are all separate things. Monks enjoy the later two, but not the first.
There are three ways to get proficiency with a given weapon in 3.5.
So, humanoid Monks manage to fall through the cracks on all counts, an
This is one of 3.5's most famously dysfunctional rules.
I believe the unarmed strike falls under the Simple weapons, so you are proficient with unarmed if you are proficient with simple weaponry. Monks are not the only proficient characters, but they do get the Improved Unarmed Strike feat, and improved unarmed damage class ability. Also, as BESW mentioned in the comments, unlike nearly any other weapon, you are not "considered armed" with unarmed strike unless you have the Improved Unarmed Strike feat.
EDIT: Since the three classes that do not get automatic simple weapon proficiency (druid, monk, wizard) do not seem to mention any Unarmed proficiency, I believe it is safe to assume that every character is automatically proficient with his own unarmed strike (See KRyans more complete answer for information about that). However, extremely RAW, these three classes are not proficient with their own unarmed strikes. Funny for the monk, isn't it?