I may be having a different experience than everyone else. I spent every waking minute of GPNW 2011 introducing people to each other (and the same thing to a lesser extent at GPNW 2010, when I was an actual organizer).
I think it's really that simple: if you know people, even two people, play party host and introduce them to each other. Introduce people even when you're pretty sure they do know each other. It can't hurt! Give them some point of conversation. Say where you know one or the other from or mention some cool game you played in with either of them and boom the ball is rolling.
Likewise, *tons* of people came up to me and said "hey, I want to meet X (but I don't know what they look like), can you point them out to me?" and away we'd go to hunt and introduce! It was lovely.
Make it part of the intro speech for Friday night. We bring up the "you're all strangers and that's awesome" point every session of Story Games Seattle. Encourage people to sit at random tables (like the food Donut idea Lee mentioned, which is awesome, though really it's the food Lottery).
Please don't turn the Friday Night Feast into a game. It's one of the few times people can mingle and talk.