I would love to see it. I continue to make more extensive use of these over time. Two years ago they didn't matter, but now, especially post-Florida where they dominate the Google SERPs for eCommerce, they are extremely important.
My own guess is that they will account for 2.5-10% of a retail ecommerce site's traffic. Anyone else care to venture what % of their conversions/traffic comes from Shopping.com, NexTag, BizRate, PriceGrabber?