The new WordPress.com utility can be found via the Tools section within the Dashboard. All that's required to run the import is your Posterous host name, user name and password, and it can import all of the following: posts, tags, comments and images.
On the flip side, Posterous has supported the inverse functionality -- import from WordPress (and other blog platforms) -- for several months, along with an autopost to WordPress option.
Now that you can get your content both in and out of Posterous and WordPress, it seems as if there's a battle brewing between the two services, which serve overlapping purposes but offer unique advantages. We see the value in both, but we're curious if WordPress' we're-just-as-dead-simple-as-the-other-guys strategy will convince you to switch over. Share your thoughts in the comments.