This is commonly known as "typosquatting" i.e. buying a domain name with the expectation of siphoning traffic off of the legitimate site by snaring people who can't spell well. There have been a number of court cases as a result of this kind of behavior - most were decided in favor of the site owner. Ultimately, it's your decision.
While this tactic may get you some "free" traffic, it's also a great recipe for a lawsuit. If the .com domain is trademarked, it's better to steer clear of trouble. If the .com domain is "generic" (that is, it is a dictionary word which is not trademarked) then you're safer. Ultimately, it's your decision.