Previously, he had a plan where you paid a membership fee (included when you bought a T-shirt from him) and then got access to the full RSS feed. A feed was available for free, but it was partial content only. Now, he says, confusion about which feed is which, and RSS readers without HTML authentication (most notably Google Reader, and the .Mac reader for iPhone that David loves so much-- what's up with these popular readers not providing a feature that supports the content producers that supply them?) have convinced him to try out a free full feed for a month. Additionally, the free feed will be supported from week to week by sponsors.
If you haven't signed on to DF's feed yet, now's your chance. And if, like us, you find it interesting and informative, might as well put a couple bucks in the bucket and pay for it anyway-- Gruber's definitely earning it.