When posts are set to contain multiple categories, they never get posted over in the forum. They also don't show up when you go to "Export Posts".
Here's what happens.
* post w/ 1 category mapped to forum -> ok
* post w/ 1+ categories not mapped to forum doesn't show up (good)
* post w/ >1 categories w/ 1 mapped to forum doesn't show up (BAD)
* post w/ >1 categories all mapped to forum doesn't show up (BAD)
An OK behavior would be to loop through the category array until you get the first match to a category->forum mapping ... but that's not currently happening.
I suspect I'll end up hacking the solution to this myself, but I wanted to see if anybody else has had this problem and created a solution already.