Hi. I was talking only about *qué* and *cómo* as ways to ask "what did you say?" As Gerald Erichsen says (on about.com), *cómo* is not used to mean "what" except in that circumstance or to express indignation/astonishment. I'm surprised he says that "in some areas" *cómo* is considered rude as a way to ask "what did you say." Just about every book I've seen about Spanish suggests *cómo* as the best word to use, unless you're in Mexico, where you might instead use *mande*. I guess I'd simply say that in situations where *cómo* is considered rude, *qué* is likely to be seen as even more rude :D. Such circumstances might call for an _extremely _polite statement such as *Disculpe, podría repetir lo que dijo, por favor*.
For what it's worth, the dictionary on WordReference.com offers a number of definitions of* cómo*, including this one: *(cuando no se ha oído bien) ¿cómo?, what? *. And in the WordReference forums, several people compared cómo and qué and offered comments such as the following three:
"¿Qué?" is EXTREMELY informal, perhaps even incorrect. You might say it that way to a friend or close family member in casual conversation but it is more corect to say, "¿Como?" or better yet, "¿Mande?"
I have heard people say, "¿Qué?????" as in "What the heck are you saying?" and mostly they are younger people saying it. ¿Cómo? seems to be the most common word.
I also believe '¿Cómo?' is more adequate than '¿Qué?' unless in a very informal context, as it is short for '¿Cómo ha dicho/dijo?'