actually, looking closer (and thinking about something that "just pops up" in the garden), could it be potatoes? if you google "potato plant berry" & look at the images, they look a lot like your plant there...
Solanum Nigrum grows as a weed in California so could potentially pop up anywhere.
The berries just seem too close together and too small to be a potato berry to me. The leaves seem right for the variety of nightshade I have seen growing as a weed here in Sonoma County.
The flower shown in the close up pic looks very much like these. Although I know most nightshades have flowers that are very similar (including potatoes and tomatoes)
http://web.ewu.edu/ewflora/Solanaceae/Solanum nigrum.html
Potatoes, tomatoes and nightshade--Leaves are bad for chickens. Chickens can eat tomatoes but not the leaves. Like wise with potatoes, except for green skin--too much Selenium. We are not supposed to eat green skin either.
Jason will pop in after work and let us know what he thinks.
