The extension of the domain does not matter as far as trademarks go. You should avoid registering any domain that is a trademarked word or phrase. You may also get into trouble if the trademark is contained in the domain name.
I used to have some domains that had trademarks or were very close to trademarked terms. What happens is generally that they tell you to stop using the domain, they take the domain, or they sue you into a bottomless pit of financial ruin. The last option doesn't happen very often luckily.