No, a SKU is a number used by a company to identify an item. The number below a UPC bar-code is the UPC code in numeric form.
Yes, some companies do use the UPC as the SKU, but in inventory terms, the SKU is basically the in-house item number (for example, Staples and CompUSA have differing item numbers (SKU's) for the same item, but the UPC is the same).
Hope that helps.