I have a feeling many shoppers loose money when they add "just a little more," in order to meet the requirement.
For example, a site which offers free shipping on a $50 cart may be tempting enough to someone who was originally only going to buy $30 worth of products, to add the difference in order to qualify.
When I shop, I keep track of which sites always offer free shipping at certain amounts (or who have specials). I then save up and buy everything I need from that place all at once, in order to truly benefit from the free shipping. For example, Dick Blick.com (art supplies) offers free shipping on $200 or more. I wait until I need $200 worth of art materials, then the free shipping is an added perk.
Planning a purchase can take advantage of free shipping offers. Spontaneous item-adding to meet a free shipping minimum is akin to the impulse aisle near a grocery store checkout. You're faced with additional items at the end, and you grab some even if you're on a budget.
When it comes to sites where the shipping is always free, the consumer is likely paying the difference via the cost of the items. Shipping isn't really free in the end--but it's handy marketing.
Some places also offer a version of free shipping, along with paid shipping. DeepDiscountDvd is a good example. Anything can be shipped for free. If you need the item faster than, say, a month, you'd better pay for shipping.
As a buyer, I DO pay attention (and use) free shipping offers. As a seller, in the past I've run temporary sales on shipping to boost orders (it usually works). On the new site, we're offering free shipping on orders of $100 or more. The site hasn't been open or functional long enough to know if it's going to help sales yet. We'll see.