Minecraft creator Notch just put up a shot of the forthcoming Halloween Update on his blog , showing the new Hell environment. It's an alternate dimension you can switch to for fast travel: a few steps in this world translate to a few miles in the other when you switch back. The shot he put up is pretty dark, so here's a brighter version that shows the huge, cavernous shape of this new area better.
There's some kind of sand-coloured substance - possibly sand - in the upper left corner. Notch says he's adding at least five new material types in the update, one of which is presumably the reddish igneous rock here, and the yellow stuff may be something more valuable. If it were sand, after all, it ought to be falling from there.