If I have been up and running a while and the .com isn't being used and never has been, can the .com owner then setup a site similiar to mine on the .org to try to siphon traffic? The domain in question is not a common word and is not in use anywhere other than my site.
I mean it is a possiblity but I wouldn't worry about it. You are the original and you will rank the best for the terms on search engines. I would keep building your site.
You still able to rank better than the .com if your site quality and related to the domain name. Al thought .com may help them rank more easily but content is a king which will help you to rank on top of him.