Sachin's mentioned planned premium features in the past, so I think that's a good bet. They could also do one-off deals with big companies, like they once did with Coca-cola and Loopt recently did with Virgin. They've got a good brand and smart founders, I expect they will do just fine.
Honestly, I startups often say "premium features" because that's something that users understand and aren't scared by.
Posterous has raise $5M, so they really need at least a $40M exit, preferably bigger. Are they really going to make enough for that kind of exit from a small percentage of their users paying, say, $5-10/month for premium blogging features? Doubtful.
I think their plan, as well as that of many other startups, is to grow as much as possible and then either find a way to monetize their large number of free users or get bought by a company that wants some combination of the users, the employees, and the technology. But the average user won't be reassured when seeing this in an FAQ:
"It's free, what's the catch?"
"We want to get as many users as possible while making no money, and then we'll find some way to either bring in revenue or sell the company"
Users would worry that Posterous either will die one day when they run out of money or eventually realize they need to make money and ruin the product. But "you'll always be able to keep your same free product, but we'll add some even better features that cost money" isn't scary.
This isn't unique to Posterous. I don't remember which startup there was, but I saw "premium features" on the FAQ of another startup and literally laughed out loud because I knew that the combination of the amount of funding they had raised and the small amount that most users would be willing to pay meant that that was a lie.
