Do it or document it?

I come from a corporate IT background that involves lots of formal up-front documentation. I now have a startup project I want to work on, and my head is telling me to start writing documents, preparing detailed designs, etc. But my heart just wants to get in there and build the thing. I wondered how many 'successful' services knocking around today didn't start out with a written design - I'm thinking along the lines of Twitter, Wesabe, etc.