There's too much emphasis in this industry on registering extensions other than .com .
If we look at popular websites like Facebook in example:
http://www.alexa.com/data/details/traffic_details/facebook.net , there really isn't all that much of a traffic leak.
Important for a large company who doesn't want confusion among customers, but for a small company or individual owning the .com, someone developing the .net will more likely than not increase traffic to your site.