Cool. What is Posterous's business model?
They've now got a solid product and userbase. The business model could be any of advertising, affiliate marketing, lead generation, premium features, ecommerce.
That said, I agree. I think it's great that they're taking some time to see how their product evolves, especially when they seem to sincerely want to "change the game." One thing YC companies seem to do well is that they have the patience to take the time to explore those markets alongside their users, instead of dragging them into a business plan that detracts from how users want to use the product in their daily lives.
EDIT: Whoops, this answers part of the question, from their FAQ: Yes, Posterous is enthusiastically a free service. Later, we'll be adding premium features we know you'll love, but there will always be a useful free version you can use.