We actually did exactly what you're asking here a few weeks ago and applied to YC this round with it.
We built it in node.js and MongoDB. While node and mongo are young projects we have found them very stable and extremely fast. (we noticed a lot of problems reddit had with scaling and we were interested in the stack)
Our decision to build Exipe in node/mongo has been a bit controversial but we've found no problems thus far with it ... ~2 weeks out with slowly growing traffic. Check out a few of my latest blog posts here about the experience: