For example, registering mcdonalds-diving.com isn't likely to be a problem. Yes, McDonalds is trademarked by that well know fast-food chain, but if your name is Mr. McDonald and you're in the business of supplying diving equipment, this shouldn't raise any problems.
Registering mcdonalds-restaurants.com is a different ballgame though, as it uses the name of the previously mentioned fast-food chain and also the word restaurant, which is obviously the area in which the fast-food chain operates. It would be easy for them to say you had registered the domain name in bad faith and they would have a strong case against you; it may be hard for you to prove otherwise.
In short, you need to use your head! Registering a domain name that is very obviously similar to a trademark may not be the best idea, as some companies are very protective of their trademarks when it comes to domain names!
R.