A now-edited PlayStation Blog post suggests that the multiplayer mode in Uncharted 3: Drake's Deception will be offered up as free-to-play content (via GamingEverything). The post specifically mentioned "Uncharted 3: Drake's Deception Multiplayer (Free To Play)," though the fact that the post was later updated to remove that detail puts this news squarely in the "credible rumor" category until we hear otherwise from Sony.
Naughty Dog promised "some very exciting information about [Uncharted 3] multiplayer around the start of next year" in a late-2012 blog post, as Joystiq notes. There's been no confirmation from Sony, though if things are far enough along that a mention made its way into a PlayStation Blog post, then you can likely expect to hear more soon. Assuming, of course, that the information is accurate to begin with.