I think they do some checking in the headers. It's not impossible to masquerade as someone.
But how many people want to maliciously post to some random person's Posterous blog? Probably not too many.
But how many people want to maliciously post to some random person's Posterous blog? Probably not too many.
They wouldn't necessarily even have to get you to click on a link. There have been numerous attacks against image processing libraries.
Maybe Smear Campaign artists might take to it. Or spammers...