Depends on whether the .com ever registered a "mark" with that domain - if they did they could easily take any domain with that same name - an example is the "for dummies" trademark - they send out domain infringement letters all the time and have gone to the governing body on more than one occasion to get domains taken away.
If its something "general" in nature of course none of this applies - if it is an operating website, then ethically the practice has been to not register the .net of the site although quite a few scammers out there make it a regular practice - I think there was a recent case here where the name of the other website was only part of the guys new domain and he was pretty much dragged around here.
In the case of an affiliate paysite there are some sponsors that love to let you register the .net/.org etc and make them money if youre good with SEs
- then there are some that would probably try to sue you
