I'm having this issue as well. I created an outfit and followed a tutorial on how to export it, but it won't work in-game. When the GECK renders it while trying to make the item for use in-game, the GECK crashes.
This is the tutorial I followed: http://wiki.tesnexus.com/index.php/Creating_an_armour_for_Fallout._Part_1
I've never been able to get anything to properly work after exporting it for some reason; even things for other games, using different modeling programs.
In this particular instance, I keep getting this message that the export failed because certain vertices were not linked to a body part (I believe it said), then highlighting said vertices.
I got it to "successfully" export once by deleting the section of the armor associated with those vertices, but it still wouldn't work and caused the GECK to crash like I said.
As far as I could tell, the vertices were indeed linked to a bone, and properly weighted.