From Terminal Gamer:
Title Update 1.04 Fixes "Error Syncing Player Data" Issue
January 27, 2010 - A significant number of Uncharted 2 players ran into an error which would prevent them from being able to play Uncharted 2 online. Today, Naughty Dog released "Title Update 1.04?, which fixes the "error syncing player data" problem that prevented certain players from being able to connect to the multiplayer portion of the game, winner of our "Game of the Year Award".
The fix required Naughty Dog to roll back the accounts of those affected by the error to Level 1.