The strength and scope of trademark protection is based on the type of trademark and the class of product it covers. Trademarks in descending order from strongest to weakest are fanciful, arbitrary, suggestive, descriptive and generic. Google is a fanciful (i.e. made-up word) trademark and has strong levels of protection. It is also famous which gives it additional trademark rights. In certain situations, there may be legitimate reasons to register a domain incorporating someone else's trademark. Just make sure that if you do so, you have a legitimate reason and evidence to back you up in the event a dispute arises.