I kind of wonder if there is a middle ground. There are two different posts on the front page right now: one deriding the idea of co-founders and one from someone desperately seeking a co-founder.
My (worthless) advice is to let it happen naturally. Neither seek out a co-founder nor refuse co-founders who approach you. Just try to make friends with similar people who are interested in startups. Talk about ideas. It definitely helps if you have known these people for a while before starting a business with one. I can't even imagine going in on a business venture with a total stranger.
This sounds like good advice to me. This YC requirement has led to something akin to "cargo cult cofounders" - getting someone else involved because it's "better that way". While I tend to think it is better to have one, it has to be the right person, not some random dude who you barely know. It's sort of a catch 22 in that it's got to be someone you've already known for a while when you start looking or discussing the idea of doing a company.