I just called the Olay folks and the $15 gift card that you'll get for the current rebate is basically like an Amex gift card. Its not limited to Olay products and you can use it wherever.
I called because I wanted to know if I could submit my Olay receipts for this rebate and also for the spend $50, get $120 coupon book rebate. She could only speak for the Olay rebate, and that one requires an original receipt. I also called P&G just now and they require an original receipt as well, so unfortunately, no double dipping on this one. Shucks, wouldn't that have been nice?