Sounds like the easiest thing in the world?
Well, much like when I migrated from Movable Type to Posterous via WordPress, it took some work.
Since the Posterous Importer plugin for self-hosted WordPress wouldn’t stop coughing on my Posterous site, I first signed up to WordPress.com (for free) and used their native and better maintained Posterous import plugin to suck all my posts and media.
Then I immediately exported all of it using the Export menu and then imported it all back into the self-hosted WordPress installation before deleting the WP.com intermediary blog.
I’ll pass on the setup necessary to keep the nice blog.aka.me sub-domain but it’s all working nicely now.
So nearly 2 years ago I did:
MT → WP → Posterous
and just now:
Posterous → WP.com → WP
* Update 1
hehehe it was kinda too easy, right?!
Turns out I am missing half my posts and comments. Duh!
Looking into other solutions.
To be continued…
* Update 2
If you notice a discrepancy between the number of posts on Posterous and WP.com, you probably have character encoding issues in some of your Posterous posts (I did, from the original MT import years ago). After fixing them, I was able to import the remaining posts. All done!