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Many .orgs are not for profit, and make the false economy of not buying the .com when they can.
Taking advantage of that is not illegal - but is unlikely to make you many friends.
Using the .com any other way is, of course, not an issue.
But if you go through with it, you also may want to consider getting a trademark registered yourself because as time passes it would become harder and harder to get it expunged and the potential for trademark problems will be minimized.
[edited by: Webwork at 4:44 pm (utc) on Mar. 3, 2007]
[edit reason] Charter [webmasterworld.com] [/edit]
I do pro-bono work for an area organization (using a .org) which has been around for about 8-10 years, with a DMOZ listing and some nice search rankings to show for it. A while back I discovered a parked page on the .com with keywords relating to the organization's very "niche" business.
I doubt the .com owner is getting much out of it - by now the group is heavily branded on the .org and anyone typing it in is more likely to click their BACK button than on those links, but still (and I'm saying this as someone who doesn't have a problem with GENERIC parked pages) its obnoxious.
As I recall, one of the group's members is an intellectual property lawyer. One of these days I may have a little chat with her about it ...