Stamen has been working with tiled map data for several years now. We started with the Cabspotting project, refined the work with the hosting of our own free maps for use by all and most recently released some new base maps for CartoDB to visualize data on top of. At this point the technology stack is fairly well-understood. Google and others are increasingly using vector data to serve their mobile apps. We're using tile-based tools to do design work that fits into other, evolving rendering toolkits.
(originally titled "It's Custom Cartography for MapZen Time, Motherfuckers" h/t McSweenys)