Many low budget games (like Zack and Wiki) have such small development costs that they can break even after less than 100,000 units are sold to retailers; if a publisher made $20 off of each copy of a game that was sold (which is a reasonable estimate for a $40 game) a $5 Million game, which would be middle of the road for a Wii game, would require sales of 250,000 to break even.
I think it is fair to call any game that is unprofitable a flop ...
A game like Zack and Wiki is going to be very difficult to categorize in either category though because it will likely end up bringing in a ROI similar to some of the better selling games of the year, but at the same time will (probably) not sell more than 500,000 units.