It's not a ff mile offer, but MCI is offering a round trip ticket on Sun Country airline.

You receive a travel voucher after three months with MCI. The voucher is good for a free flight anywhere Sun Country flies in the US. Sun Country is based out of St. Paul/Minneapolis and recently expanded their services to several east coast cities. They also fly to the west coast.

I signed up this morning. 1 (800) 754-0037 and ask for the Sun Country Partner Promo.