SOLUTION that worked for me with Mac OSX 10.7.5: I'm having the same problems as everyone here with Lightroom to Dropbox export with zero bytes. (1) Pause Dropbox syncing. (2) Publish to Dropbox from Lightroom, as you normally would. I publish high res and low res images to different custom folders in my Dropbox. I don't think it matters where the folder resides in Dropbox. I am NOT using a plug-in...just have my Dropbox folders set up in Lightroom's Publish Services pane. (3) Let Lightroom complete the export. (4) Resume Dropbox sync. Done.
This worked for me...everything back to the way it was before this strange problem occurred last week. I hope this works for you. Cheers.