Organic materials, such as trees, grass and plants, regenerate or can be replanted. Metals, dirt, sand and stone do not, but since the size of the procedurally generated Minecraft world is IMMENSE (think Elite II: Frontier- and Daggerfall-like immense, theoretically unlimited, but eventually the world file will grow to massive for your storage or hardware), there will always be more material, provided you are ready for a hike. (And hiking might be an overstatement. Since you can always dig downwards and outwards, the material available in the first few hundreds square meters of your starting area alone can last for weeks.) That said, I will wait for survival mode (enemies and animals), and the refinements of the Adventure updates for Minecraft for Mac/PC, though with Notch's stated intention of creating a light portable version (a, to me, completely mystifying sentiment), I' not sure we will ever see this.