tjpick thanks for pointing out how you can get more than 50 posts out by using the page parameter. Yes, the documentation is not very clear at all and we need to fix that.
rizzn, it's true that most of our users wouldn't be able to use the export API since they are not programmers.
But we're relying on programmers to write the shims needed to go from Posterous to Wordpress, Posterous to Tumblr, Posterous to Blogger. We simply don't have the resources to do this, so we wrote export to be as generic as possible.
Other sites do export by letting you pull down an XML file with all posts. We could do this, but then what about the media? Where should we put that?
The point is, we always want to let you get your info out. We have assured our users (including Scoble and Rubel personally) that they will always be able to get their data out, including the original files they have sent us, if they ever need to.