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?

There are A LOT of people in WA. St. whom I've heard say 'warsh' and my Dad says 'crick' for 'creek' and he's from S. OR. I've also heard a fair amount of people who say 'febrar' for 'febuary'.

I say (and most people in the NY state tri-state area) say Febyoowery for February

I too live in Washington, and can confirm that many people pronounce it 'warsh'. I am one of those who pronounce creek 'crick', and have even won a Knowledge Bowl based on this pronunciation .

I'm not just familiar with the "warsh" accent. I have the "warsh" accent. And I HATE the word "warsh," but I have to consciously say /waS/ rather than /war\S/, because it's the accent I'm surrounded by.
(I'm from Texas, incidentally.)

Another good South-ism, by the way: "fling a cravin' on" means "to cause another person to want a particular food, either by eating said food or by talking about it." Thankfully, my vocabulary has managed to avoid acquiring this little phrase, mostly because this is an old-folks-ism.

I've heard of "bananer", but not "warsh". I also thought that some non-rhotic dialects of English tended to stick r's where they weren't.