The US State Department has extensive and arguably alarmist warnings for the states of Mexico, read them here: http://travel.state.gov/travel/cis_pa_tw/tw/tw_6033.html
But for Oaxaca they have no warnings at all. Indicating it's generally very safe.
Try to not get too frightened by media reports of what's happening in Mexico, the truth is that very, very few of the 20 million+ tourists who visit every year have been deliberately targeted, and usually not in tourist centres such as Huatulco.
You can read more of our assessment of the situation here:
Phil at the safety hub. about 4 years ago