I don't really have a theory as to why this is mispronounced as such, but here are my observations of whom I've heard it from:
It appears that mainly the older generation (appx. 40+ (no offense to anyone in this age group
)) say warsh. I really haven't heard it from the people my age that I know. My father is from Iowa, and both he, his parents, and his cousins (again, same age or older), all mispronounce this. My older family from Texas all say this. My much older cousin uses this mispronunciation, I believe he grew up in the southwest, Arizona, New Mexico, etc. My friend's father was born in New Jersey, and spent several years in I believe his early adolescent years in Peru. He too says warsh.
So if I may be so bold as to suggest this appears not to be related to geography as demographics (more specifically, age grouping). Does anyone else find that this may be an accurate theory?