Oh and D don't do anything stupid like Keyword stuffing, scraping, fake content or anything else that can get you banned...well unless you are Mahalo...then you are off the hook.
Personally I don't think YC startups actually need SEO link building. The mere fact that they are a YC startup, means that they'll get on the front page of Techcrunch, ReadWriteWeb, VentureBeat and dozens of other huge blogs with a ton of PR when they launch. A single post on Techcrunch is worth more link wise, than a million PR1-3 blogs posting about your latest link bait strategy.