[synotic]

Depends whether you're innovating in the frontend or the backend. The user interface for the iPhone was completely designed before any of the frontend was coded. There's a great comment by tlrobinson somewhere about how the iPhone was designed and developed but I can't seem to find it.

Also consider New Twitter. I don't understand how you'd start coding the HTML and the JavaScript without understanding the visual design for how the dynamic, two-pane interface was expected to work.

But if you're building a search engine, then you can probably put off the design until later. The analog is that you really wouldn't (or shouldn't) start writing code until you had an idea (a design or an architecture) of what you were trying to write. Same way you wouldn't start to code an interaction (HTML+JavaScript+CSS) if you had no idea what that interaction looked like.