I've just done the same thing. Avid mechs up front on an XT Rhyno Lite wheel and XT V brakes on the back (partly because my older frame doesn't have the bosses for rr disk and partly for cost). My reasoning was exactly the same as yours; 70 to 80% of braking is up front, front disk looks cool, and the v's are plenty strong to lock the back wheel with some eventual fading. My bike is in the back of my truck right now and I'm planning on an early escape from work today for my innaugural ride with this brake setup. I'll let you know how it goes. BTW what should I know about disk brake break-in periods?