I agree with Calistoga. There's a guy up the street from me who has a large Alaskan Malamute and I was CONSTANTLY cleaning up after his dog. It drove me nuts because I never let my own dogs out in the front yard...only in back so when I mow the lawn, I typically shouldn't have to scour the front yard for dog messes before I mow, as I fully expect to have to do in the back.
One afternoon I happened to be leaning out the front door to get the mail or something just in time to see his dog finishing its business on my front lawn and as he was about to walk away I rather loudly said, "Excuse me! Do you mind cleaning up after your dog, please?" He looked around embarrasedly and said he didn't have a bag. I grabbed a few from the roll I keep near the door and wordlessly held them out to him. He red-facedly cleaned up after his dog and I've never found another mess of that size on my lawn ever again.
On the flip side, I was out walking with Radar and Jagger one beautiful autumn afternoon and one of them stopped to do his business on the grass median in front of someone's house. I stood and waited for him to finish, then turned and saw a woman glowering at me from the front window of the house we were in front of. I didn't acknowledge her in any way (just yet, I don't think she knew I knew she was there), removed a baggie from the roll holder (every leash I own has one of these attached. That way I ALWAYS have baggies) cleaned up the dog poo, then turned and waved at the woman, who smiled and gave me an approving nod. I think she must have the same problem I do, and wanted to make sure I was cleaning up after my dogs.
Now the only problem I have is the guy with the two Cairn terrier mixes. He walks them on those accursed, should-be-banned retractable Flexi leashes, and lets them wander wherever they want. During the winter, I see their tracks coming all the way up onto my front steps, and my house is set quite far back on the lot. Who DOES that??