Still great advice, six years later:
Ideas are in the air. There are lots of people thinking about—and probably working on—the same thing you are. And one of them is Google. Deal with it. First of all, realize that no sufficiently interesting space will be limited to one player. In a sense, competition actually is good: especially to legitimize new markets. Second, the specialist will almost always kick the generalist's ass.
Evan Williams, Twitter co-founder. His original post.