If you are moving from Posterous to WordPress, like I did, and you write your posts using non-ASCII characters (such as those very weird from el idioma español) you will notice that everything becomes garbled after you import.
The solution is actually quite easy. Inside the posts/ folder of your zipped Posterous backup you’ll find a bunch of XML files. Just concatenate them together, prepend an XML header (like this, but you’ll find the exact one for you reading your wordpress_export_1.xml file) and append </channel></rss>.
Finally, you have to cut all the carriage returns. I used Cygwin’s cat for the first part of the process and Notepad++ for removing the \n’s. That’s it.