I think that the most relevant example is the use of the word "maverick" by John McCain and Sarah Palin. (I saw that there was a post about the word specifically at one point.) It seems that the definition of the word has been lost, become meaningless, and is being appropriated by McCain and Palin and thus redefined.

But I am also interested in what happens at a more basic level. Like when you say a word over and over again out loud and it begins to sound different. Eventually it becomes just that- a sound. The meaning dissolves.