Can a monk take Improved Natural Attack (Monster
Manual, page 304) to improve his unarmed strike?
Yes. As stated on page 41 of the Player’s Handbook, a
monk’s unarmed strike “is treated as both a manufactured
weapon and a natural weapon for the purpose of spells and
effects that enhance or improve either” which includes feats
such as Improved Natural Attack.
Barring multiclassing, the earliest a monk could take this
feat would be at 6th level (due to the base attack bonus
prerequisite), at which point her unarmed strike damage would
improve from 1d8 to 2d6 (which represents an average increase
of +2.5 points of damage). The same monk at 20th level would
deal 4d8 points of damage with her unarmed strike.