The Best Way to Open Content Paywalls?

I just stumbled across this, and have planted it firmly in my "things I didn't know, but find really interesting" pile - the WSJ's old free content model:

But what WSJ.com used to do was to offer a backdoor to free content for another class of consumer: the social media maven. Paying subscribers could make content free to others by clicking on an icon that created a URL for a free version of the story that they could use for blogging or to submit to sites such as Digg or Yahoo Buzz. The deal was essentially this: these often influential word-of-mouth generators could trade reputational and attention credits for free content. The content would be part of the online conversation, not walled off behind a paywall, and presumably some fraction of those who followed the links to free content would recognize the value in the premium content around it and subscribe. A very nice Freemium model, in other words.
And that strikes me as a really bright way of charging for some very high-value content without completely isolating it from the conversation on the web. Most of the models I've seen involve the journalists choosing which elements of content to free up. The idea of allowing the paying subscribers to do it, so they can link it/discuss it on their own blogs is compelling - provided, of course, that you have a base of social media active people in your readership.