I'm going to be in the minority here and as a marketer I am probably biased, but I think you should have a marketer or a business guy as soon as possible, when you are designing your product, writing sales copy, setting pricing ,deciding the angle/niche you target and so on. These are are critical early marketing tasks. By marketer I don't mean one of the millions of narrow minded seo guys. I mean someone who can develop a deep understanding of your target audience and the marketplace and how your product fits in
I agree you need someone who understands your target market from the beginning. However, I think that for some markets, a conventional 'marketing guy' would be the wrong choice... for example, I sell infrastructure to SysAdmins. Yeah, if I wanted to sell infrastructure to managers, I'd need a conventional marketing guy. But for my current target market? I am pretty close to the optimal 'marketing guy' because I'm building a product that I would want to buy myself, and I'm selling it to people who buy it for the same reasons i would buy it.
I think it's essential to have someone on the team who understands the target market, their needs, their culture, and their feelings. It's really difficult to know how to build something you don't want for someone you don't really understand.
