Originally Posted by matt in france

ignore what Continental says, that's specifically for CO Onepass members...

In *A, it's always the OPERATING Carrier of the Flight that counts for Mileage accrual, so M&M will give you 50% Mileage on Flights operated by UA, and 100% on Flights operated by CO.
Of course, that's always assuming the Flights actually book into W; CO's W Fare may map into a different booking Class when operated by someone else's Flight...