Reading the comments to Jökull's Calepin Guide, I found this tip about posting to Wordpress from Calepin, via Atom publishing.
It involves using ifttt, which is a neat service that glues different web-based channels together. You set the service to trigger on a change to your Calepin feed and then post to Wordpress via their Atom Publishing protocol. I went to give it a try, but unfortunately, that feature (posting via Atom) isn't available with *.wordpress.com hosted blogs.
This method seems like a good stopgap for those who want to try Calepin without committing to it. It allows you to post once and have it get automatically migrated. It also exemplifies the usefulness of ifttt. As I mentioned before, if the ifttt Dropbox channel can just do a little bit more, then using a Dropbox-backed service like Calepin will become even more compelling.