Long-time Stencil Archive contributor Bryn threw a link my way from a Sacramento, CA government website, asking citizens to "Help educate your neighbors, Get a group together to stencil drainage inlets in your neighborhood with the 'don't dump here flows to creek' message." The PDF packet is about as legal as you can get for this stencil.
Here in San Francisco, the National Surfrider Foundation has begun to stencil drains around the City. Their site is looking for volunteers that will be "working with the local city governments on developing this program, designing stencils/plaques to be placed at storm drains, recruiting and coordinating volunteers to attach stencils/plaques to storm drains and map locations of storm drains/plaques. We need YOU for the project!"