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/