The official twitter account for Penny Arcade Expo has broken the news. "I don't want to alarm anyone, but based on current sales trends, we're going to sell out of PAX Prime 3-day badges in about ... 15 days."
I'm of the opinion that you don't need to visit the show floor all three days to have a good time. That said, I wanted to toss this message your way, on the chance that you were curious about how quickly these passes were moving. They're certainly moving, all right.
Remember, PAX Prime starts on August 26 this year. Plan accordingly!