Ok, so we already knew how to, but they didn't say when.
Effective tonight, err, tomorrow, whatever, at 12:01AM PST on October 10 and ending 11:59PM PST October 12 (that's 3:01AM start and 2:59PM end time for those who face my recent enemy, the UK), play the demo. According to the PlayStation.blog:
Log on and play the UNCHARTED 2 online Multiplayer Demo from Saturday, October 10, 2009 at 12:01 AM PST through Monday, October 12, 2009 at 11:59 PM PST. We will be giving away 10 Fortune Hunter Editions randomly (not based on stats) each day. That’s 30 total FHEs being given away over three days. Just launch the Demo, go to the online lobby, select the Matchmaking icon and join any open match. Once joining a match, you will be entered into the promotion for that day.
We’ll be randomly drawing 10 entrants each day. We’ll post winners’ PSN IDs on www.naughtydog.com and the PlayStation.Blog on October 19, 2009. We’ll also notify winners via the email address associated with each entrant’s PSN account between October 15, 2009 and October 30, 2009. Must be 18+ to be eligible. Full details and legalese below. Good luck this weekend!
That's it. Join any open match, and you're entered automatically. Be prepared - those who play the demo on occasion - to have a lot of people drop out once a match has started. Have fun. I know I will.
Source • PlayStation.blog