Naughty Dog has announced that the Uncharted 3 multiplayer beta has been checked out by more than one million different users - a number which is growing rapidly.
The beta is currently available to anyone with a PS3 and a PlayStation Network account, although when it was initially launched, it was only available to those with PlayStation Plus.
The beta is planned to close on the 14th of July, so there's still a few days left. Once it closes, you won't be able to play the multiplayer until the game releases this November.
Source: PlayStation Blog