You could buy a shirt and it came with the membership, or pay less for just the RSS membership. He delivered a personal RSS link with a random string. That worked until people moved to Bloglines, because it shared URLs among all its users and thus exposed private URLs to anyone looking for the daringfireball.com feed. At that point, he moved to password protected feeds, until people moved to Google News, which didn't support password protected feeds. This is when he began the weekly sponsorship model.
It was on a paid subscription model before the current one. Loading the site was free, but you had to pay for the RSS feed.
Or maybe it was that buying a shirt got you RSS feed access for a while? Don't quite remember now.