Uncharted 3's multiplayer goes free-to-play as a new PlayStation Network download today. Upon download, you'll be able to jump into and play as much competitive MP as you'd like for no cost at all, though you'll be capped at level 15. If that's all you'll ever need, fine. But, if you'd like to do more, various micro-transactions will let you unlock what you'd like. Ala carte, if you will.
For example, if you'd like to bump the level cap up to 25, you'll pay $4.99. Or, take off the cap completely for $19.99. Co-op Arena and Co-op Adventure are priced at $9.99 and $11.99 respectively, but if you're going to drop that much, I'd recommend buying the full digital version of Uncharted 3. The Game of the Year version includes the full single-player game, all the previously released DLC, and fully unlocked multiplayer, for $39.99.
Naughty Dog told Destructoid that the idea here is to get as many people into Uncharted 3's multiplayer as possible, helping them move beyond the already healthy 500,000 or so monthly plays they hit now. They hope that for those that missed Uncharted 3: Drake's Deception, free multiplayer might be a nice way to draw them in.
Aside from growing the community around U3, Naughty Dog says that they're also out to test the digital release waters with this launch. This is their first foray into breaking off multiplayer as a separate piece, so they'll be watching forums and listening to fans, taking feedback into account going forward.