This is ancient, but back in the 0.8 CB days, I put together a sample todo app with mongodb that was functional.
Code:
I keep meaning to update some of this, but there's a pre-built Amazon ami that should launch with a working version of the todo app against PostgreSQL, MySQL, and MongoDB here:
http://imperialwicket.com/the-mob/ . I haven't tested that in ages, but if you want to glance at something that should be working against MongoDB out of the box, that's one example (I think there are a few others on GH).
At a glance, I managed the columns differently, though I don't see that as an immediate issue. If you can post a little more detail, maybe we can troubleshoot more effectively. Also remember that if you reference the code linked above, it was working against very old CB and boss_db commits/releases. I think some functions have changed since then.
This is all to say, if it's not working then it's a regression, because it used to work properly.
-- Nicholas