This was the fourth or fifth stencil with which Banksy graced San Francisco. As Banksy's work is often politically charged, there is some speculation that this piece is in response to the recently proposed Sit/Lie ordinance in San Francisco. I don't feel the Sit/Lie ordinance is big enough to attract Banksy's attention, and think this work is more obviously commentary on the United States' immigration policy.
The metaphor seems pretty clear cut: Native Americans were here first.
Although, there's no reason this stencil couldn't play off of both issues.
Check out the detail images below.
Close up of the head dress. Notice the technique on the white layer for the furthest left feather is different from the white layer on the three feathers to the right. Curious.
Close up of the arrow fletching. I found it interesting that he first sprayed a layer of white. My assumption is that this was done to increase contrast against the dark wall. I wonder if this means he tests the stencils somewhere ahead of time to make sure they work. Maybe he's done so many now that he can just preempt any potential issues.
Glad I took these pictures when I did. Last time I went past this spot it was covered with idiotic tags. I think this is my favorite SF Banksy (followed by Peace and Love). You can see it for yourself at Mission and Sycamore.
You can also check out higher rez images at my flickr page.