Tradename violations and domain names?

So what's the rule regarding using a company's name as a part of a domain name? For example, I see sites like "paypalsucks.com", "verizonsucks.com", "attsucks.com". However, I have to ask whether these domain names are in violation of the trademark the company has on the names? The reason I ask is that I'm thinking about creating a similar site about another company, but I want to do it to the letter of the law and not get whacked for violating someone's trade name in the URL. Is there any precedent for a "____sucks.com" or "_____complaints.com" site being sued or trademark infringement?