Collis Ta’eed’s company, Envato, is in the process of launching another FlashDen-style site. Known as Theme Forest, it will be a marketplace for web templates. Straight XHTML/CSS, or built for specific CMSes, the themes will sell from $5 to $75 (the staff set the price).
From what I’ve heard about Theme Forest, and sister sites FlashDen and Audio Jungle, you could probably make a fair bit of money off a decent theme.
The site is currently in private beta (and I’m not in the program ), so details are fairly sparse, though I’ve dug-up some details, and you could get an overall idea of how the site works by visiting FlashDen.net.