It happened in Rancho Bernardo (San Diego County), California when a Minuteman showed up armed with a camera to take pictures of a day laborers spot next to a gas station. This area usually is teeming with as many as 75 workers from Mexico on a daily basis.
See the stream of illegal workers coming out of the van here.
See the attack here. The Minuteman was struck three times and one of the ‘they’re just here to work’ types brandished a knife at him.
Thanks to Rick Roberts for the info and video.