The euphemism treadmill.
I find fascinating the process by which an acceptable term becomes a pejorative, followed by the new acceptable term becoming a pejorative, etc. There’s a great description of this process on Wikipedia:
[A] neutral (non-pejorative) term may grow to become pejorative. This phenomenon is called pejoration. For example, the term mentally retarded was originally used as a euphemism, as had been moron before, itself a euphemism for idiot, in order to avoid true dyslogisms such as feebleminded or half-witted. But it quickly grew to have a pejorative sense of its own. Another example is the use of the word cripple being replaced by handicapped. Both of these are considered pejorative with the term “physically challenged” as the current euphemism. This same progression, from neutral to pejorative, is happening with the words challenged and special, used in the same sense, today. The term “disabled” is now seen as the correct euphemism for people with both mental and physical challenges. An even more ‘correct’ form that can be used is “person with disabilities.” On the horizon, a new euphemism, “neurologically challenged,” appears ready to take the place of these terms should they also become pejorative. Language writer Steven Pinker has called this process “the euphemism treadmill.”
05/27/2011 Update: Jacob Dix provides some illuminating information on the topic.