So I dl'ed and ran http://fallout3nexus.com/downloads/file.php?id=1991 bittercup companion mod without any problems. I then made some slight edits to Bittercup's appearance in GECK which somehow conflicted with the mod. the companion mod's dialog and the new face show up fine ingame, but bittercup won't follow me. She just walks back to or stays in Big Town. Anybody know how I can edit her appearance without conflicting with the mod?

Set the companion mod as active, change the face there. Otherwise your changes override the mod's changes.