IANAL but it's my understanding that if the two sites in question are similar then the guy with the trademark would win. However, he would be entirely reponsible for defending the trademark. There is nothing illegal about trademark infringement.. but if the guy decides to sue you that's his right. The courts just won't automatically protect it like they do copyright.
In other words.. if he knows that you have the site which infringes on his trademark .. and he doesn't do anything about it .. and then sometime down the road he decides to sue .. if you can prove he knew you were infringing a year earlier then he's screwed because he decided not to defend it then.
That's my understanding of the law anyway .. I could be wrong.
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