It could be an image of Christ, of a nun or just a woman looking up. The use of only one color, with a great emphasis on negative space caused me to stand and stare for some time.
In the upper left-hand corner there's a small signature that looks like a geometric cube shape with "C215". Without delay I realized that the person responsible for this piece is serious about their art. They've chosen a somewhat unconventional form, but the quality is there, and deserving of a signature.
I went hunting around the internet for more stencils with "C215" and found out that the artist is quite accomplished and well-traveled - he lives in Paris and his work is found in cities all over the world.
Here's his MySpace page with many more of his creations. Most of his pieces are literally "street art", and many incorporate more than just one color.