Lots of smart people disagree with this. You should start marketing when you start coding. Find out who is interested, what they want, etc. When you launch, you don't want to do it to crickets.

If you have something people will search for, you can start your SEO and start adwords. 6 months down the road you are actually getting some search traffic, and your adwords have over time built up a mailing list.

If you haven't read this article by Rob Walling I highly recommend it. http://www.softwarebyrob.com/2010/10/14/startup-marketing-pa...