Stain-resistant
I have not heard any specifics as to the composition of the iPhone 4 band but I would think that Apple has chosen a steel that will hold up to being held in your hand, so long as you don't leave droplets of sweat to stay on the band for extended periods. It likely can be stained by acids found in food such as citrus fruit or strong chemicals so some care should be taken to keep it clean if you care about the appearance. An occasional wipe with a shirt or pant leg should be sufficient.
Not all Stainless steel is is equal; defined by a minimum chromium content, it's stain-resistance is affected by the presence of a variety of metals or even non metals. These additives are chosen for various properties they give to the finished steel; corrosion resistance, hardness, ductility, brittleness, etc.