<blockquote>The experiments the researchers conducted showed that people overhearing cell phone conversations did more poorly on cognitive tasks that demanded the kinds of attention we use to tend to daily activities, than when overhearing both sides of a cell phone conversation or a dialogue, which resulted in no decreased performance.</blockquote>
via <a href="http://www.news.cornell.edu/stories/2010/05/half-heard-phone-conversations-reduce-performance">Overhearing cell phone chats reduces cognitive performance, study finds | Cornell Chronicle</a>.