The GHTF (Global Harmonization Task Force) issued a draft of a new guidance on medical device classification They recommend to implement four classes for medical devices based on intended use: from class A (lowest risk) to class D (highest risk). And they give a set of rules on how to choose the classification of the devices.
Comparison with regulations
Wait, I've already seen this elsewhere. Classification of devices in Europe (CE mark) and in Canada have systems very similar to what GHTF recommends. This is a good thing to have an ongoing harmonization process. National regulations copy what GHTF recommends and GHTF copies what national regulations require. This is a virtuous circle. Maybe one day the FDA will implement this classification system.