Mobile today means more than handsets in pockets. Welcome to digital ubiquity. Always-on cross-channel services add new dimensions of complexity to the job of user experience strategy and product design.
So, yes, “mobile first,” because our users are mobile, not simply to discipline a minimally valuable product, but we have to design in parallel for mobile devices and traditional "desklap" devices.
And, actually, “Tablet First,” which means designing experiences holistically, then prototyping and implementing them first on gesture-based tablets.
The tablet form factor shares characteristics with smaller handsets and larger desklaps, so solving problems for tablets lays a good foundation for extending the experience out across the full spectrum of touchpoints.