Yes, I've had this happen to me twice in the last month as well. Both times it was a somewhat shady-looking youngster that was on foot and carrying a clipboard. As soon as I came out of my house, he nervously started swearing to me that he "didn't do nothin' to my windows". I found this odd because one of my windshields was just replaced two days before his visit and the other windshield was without blemish.
So he proceeded to show me little tiny specs on the windshield that were indicative of impending damage. I immediately started thinking either this guy has the world's guiltiest conscience or he sabotaged my windshield before he rang the doorbell. I didn't believe a word he said, but I played along.
He told me he'll give me 50 bucks if I let him replace my windshield. So I asked him for his business card. He didn't have one. Then he asked me where the nearest store was so he could get a pack of smokes. He told me he'd bring me a card the next day (which he never did) and I watched him walk out of the neighborhood - presumably to Safeway to get his nicotine fix.
I don't know if he was casing the place or what his racket is, but I smell a scam of some sort. I'm all ears for suggestions as well.