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Clearly there is a threshold for the walking speed that meets the minimum physical activity guidelines for public health particularly within the context of pedometer use. Researchers at San Diego and Arizona State Universities sought to measure the metabolic cost of various walking speeds while wearing a well-engineered pedometer. See last PDF document uploaded to this group's "Groups Files" section for an abstract and discussion of this recent paper (Marshall et.al. AmJPrevMed. 2009;36:410)
Essentially 3000 steps in 30 minutes (~3 mph for most adults) is a walking speed that when maintained is sufficient to stimulate insulin sensitization among other cardiometabolic mechanisms that are important in diabetes and CVD prevention. Does this mean that anything slower than 3000 steps in 30 minutes is not clinically effective - absolutely not but at higher muscle contraction frequencies (i.e., walking speeds) there is going to be greater PPAR and AMPK activation which are key metabolic mechanisms that underscore cardiometabolic risk reduction. It is also important to note that the 3 mph speed threshold would be faster than necessary for shorter height individuals, e.g., <5'7", or perhaps slower than optimal for those taller than 6'2" - so height and gait mechanics should be taken into consideration.