Kevin wrote:
Perhaps. I'm not an expert in language or how languages evolve so I'm not in a good position to answer your question. However, it seems to me that most metaphors are used with the explicit understanding that they are indeed metaphors and that understanding seems to supply some of their power and usefulness. Words or phrases that are misused to the point where the "misuse" redefines the word or phrase seem very different from metaphors.
There is a popularly held belief that metaphor is just this sort of overt, playful or artistic use of language (ie your eyes are deep limpid pools). However, there is a lot of research on this stuff and metaphor is far more subtle and pervasive than this. Though most of us have no idea that broadcasting has anything to do with things other than media, it is still an agriculturally sourced metaphor. Further, these unconsciously held metaphors seem to structure the way we think about the world in, perhaps, dramatic ways (ie complete cultural understandings about the structure of concepts, objects and/or actions). Our medical system is full of these; even our understanding of the immune system appears to be metaphorically mediated. That said, I think your question is pretty much "when does a metaphor cease being a metaphor?" That depends on who you ask and is a big thing in metaphor research. We may never know (and if we find out we'll probably keep it quiet until the funding runs out). Cam