Apple registered the word "Retina" as a trademark. Does it mean if you have a domain name with the word "Retina" in it, Apple can just sue your pants off?
Unfortunately, yes. Fortunately, they won't necessarily win if your domain name's usage or context don't show anything computer, mobile, or data-related (which are what their trademark is for). And should the next person ask: yes, even common words can become trademarks. It's just that they can become such only if used distinctively, rather than descriptively or their dictionary (if any) meaning.