There are a couple of threads about this on the United board that contain a lot of the details. One is titled something like Safeway miles gone?

This promo started as a test in Colorado last year and expanded to Safeway and other stores yesterday. Southern California stores come on line in March. It's tied to your supermarket savings card and you have to fill out a new application at the store adding your MP number. They automatically credit the miles everytime your qualifying purchase totals reach $250. Check out the United board for more info.