Yes. (http://ycombinator.com/faq.html)

Hmm... when I saw this in the app it seemed like a big red flag for not applying.

Would you mind elaborating a bit on what makes a single-founder worthy?

The same qualities we look for in other founders: being smart and effective.

There's a paradox here though: if a single founder were as smart and effective as we'd want, he'd already have found a cofounder and convinced him to join.

So in practice the single founders we accept tend to be people who are good at hacking but not so good at sales.

Good insight.

Have you seen the corollary in practice in YC applications or in general i.e. If a sales guy/gal is good they can convince hackers to be co-founders?

By definition if a sales person is good they should be able to convince a Woz to join them. The problem is, though, that they tend not to be able to judge Wozzes very well.

thank you. I'd reviewed the faqs, but was looking for a confirmation.

Yes, they do, and accept some of them too, according to comments PG has made here.

What are you working on?

a web app for recruitment space