If you're building a site and you know it will be complete in 6 weeks, when would you start building backlinks? Do you start adding them as you are developing the site, or wait till it's complete then start the campaign. Or does it not matter?
its much better to wait until the site is finished, you don't want google caching an incomplete, or none existent site, it could be more harm than good. I also like to leave maybe 2/3 weeks after it goes live before building links I get worried that it might not look natural to the major search engines if their done before or too quick after going live
You can start link building almost immediately, just don't submit to directories when your site is under construction. macca has a point, if you can afford, use robots.txt to keep crawlers away till your site structure is finalized.
I start right away, I throw up my first rough draft and build links before I even have my fancy final site and graphics work completed. Thats just me though. If doing social bookmarking I wait until its finished as you will often be much more successful its more visually appealing.