Thanks for this. As I read it, it's designed for porting content over the other way -- i.e. from WordPress to Posterous, rather than the other way 'round. However, the Tidy utility seems designed to clean up problematic XML files, so I thought I'd give it a go.
I took the contents of the XML file generated by the Posterous backup function -- you know, the one that none of us have been able to import into WordPress -- and pasted it into a plain text file, which I then ran through Tidy. I saved the results as a new file and then gave that an .XML extension.
Then I tried bringing that into WordPress using the import tool, and it seemed to work -- I didn't get the error message we've all been getting about a problem with the XML file. Within an hour I got an email message telling me the import had been successful.
Only thing was, it didn't make any difference. There are no new posts and no new content on my WordPress blog as a result. So, don't know what to say. Perhaps others can try it and see if they have any better luck?