Please see this photo, and the caption.

http://www.flickr.com/photos/ghb624/116598285/

The lovely spring flower is a bulb, Ipheion uniflorum, Spring
Starflower, native to South America. The foliage should
have a slight onion/garlic odor when crushed. The bulbs
naturalize well.
Each stem bears a single flower, hence the name:
uniflorum, "one flower". Sisyrinchium may have several
flowers on a stem.The foliage is also different.
Ipheion is in Lily family; Sisy. is in Iris family

Emilie
NorCal

Thanks much Emilie. I got one other response, direct to my email, but
also pointing to Ipheion:
"My first impression is Ipheion uniflorum, of the Liliaceae or
Alliaceae,
depending on how you cut the taxonomic pie."
I did an image search for Ipheion and the results confirmed your ID.
Appreciate the help solving this little mystery!
BTW, here's another image showing how abundantly it grows in one nearby
yard:
http://www.flickr.com/photos/ghb624/112046819/