Well… yes and no. It appears that during testing of iAds, Apple throws a few empty ads as a test for you to handle.
Once my app went live, ads were not refreshing, and then after a few days, they started refreshing without me making any changes. Several factors can make this happen, including number of ads available (although 1 constant ad didn’t seem like normal activity to me), and how much the user seems to use your app. I noticed that after several days of using my app myself, I was getting more and more ads.