I see no reason why not. Lots of heroes get their blades locked only to punch the villain (or elbow, or kick, or knee). It would seem that Improved Undarmed Strike is the first step on that path.
More importantly, it's equivalent of taking 'unarmed' as your free racial weapon proficiency. I'd allow this in my games.
Just a thought on the additional Weapon proficiency for humans - would it be useful to be able to choose Improved Unarmed Strike (or whatever the new equivalent will be)instead of a weapon?
Not every human fighter wants to be able to wield an exotic weapon, as sometimes their signature weapons just need the martial proficiencies. The ability to be competent when unarmed could be a more useful ability.
While i like the idea in theory, it runs into mechanics problems if the Improved Unarmed Strike remains in use in Pathfinder. Removing a non-proficiency penalty to attack is very minor compared to negating an attack of opportunity which Improved Unarmed Strike. Also, nowhere I have seen does it say any race suffers penalties to attack for using its natural weapons, which for humanoids is punch and kick. A wizard who decides to punch someone uses his BAB at no penalty. Same as if he used his staff or dagger. If there was a penalty I would be all for it.
-Weylin Stormcrowe
In my opinion, since there is no listed restriction on a human's free weapon selection, this is basically a free "weapon proficiency" feat; you could use it to select an exotic weapon, despite not having the BAB to qualify (I think I'd enforce the strength minimum for the 1-handers, though). Looking at it from that perspective, I see no reason you couldn't take Improved unarmed strike.
Seems like its now (as of release 2) only a martial weapon gained for being human.
Now it's more of a toss-up, but I'd be inclined to allow it.