Ask YC: Founders working remotely: pros and cons

Very few startups are willing to accept or allow remote collaboration; almost all startups are clear that co-founders/co-hackers must work on site, together (YC requires that someone be physically present in Cambridge or San Fran). At the same time, PG has lauded the benefits of a comfortable workplace--no set hours or attire. And even farther down the line we have folks like DHH reminding young hackers again and again that they began their company while their founders were scattered across the globe.

So what's the answer? Does it 'just depend'? Speaking from my experience, remote collaboration can be a benefit at some stages, and a hindrance at others. Also, the question arises--when two people share the same time and space, are they productive or a distraction to each other?

Thoughts?