since when is "cou" ever pronounced "q"?

"Coupon" comes from L. colaphus, meaning a blow with a fist -> LL colpus -> OF colp -> F coup, leading to couper, meaning to divide with a blow or stroke -> coupon, meaning a portion which is cut off -> English. Coupon was first recorded in English as meaning a detachable certificate in 1822.

The English word was originally pronounced "koo pon". The variant pronunciation of "kyoo pon" developed in American English, and according to The American Heritage Dictionary, was perhaps through association with words such as cube, cupid, and cute, and both pronunciations are acceptable.

Here's a pretty picture of the sound generated by coopon and cyupon: http://www.asel.udel.edu/icslp/cdrom/vol1/683/a683.pdf

And a place to put in your two cents regarding variably pronounced words: http://hyde.park.uga.edu/survey.html

Other words which I am used to hearing variably pronounced: roof, data, route.