Muxtape, a New York-based online-music startup much favored by the Tumblr set, has shut down its website, citing a "problem" with the RIAA, a music-industry organization which polices copyright. Could it have anything to do with the ease with which users can download music files from the site, despite founder Justin Ouellette's efforts to block them? The company blog elaborates, barely: "No artists or labels have complained. The site is not closed indefinitely. Stay tuned."