A new study shows no link between the two. But it's likely the issue will never be resolved completely:
Brain tumors are extremely rare and strike for mostly unknown reasons, and they can take decades to develop. Cell phones have only been used heavily for the past decade or so, which means that enough time may not have yet passed for the potential impacts of cell phone use to show up in brain cancer statistics. At the same time, cell phones have become virtually ubiquitous. There are more than 5 billion cell phone subscriptions in the world, which means it's becoming nearly impossible for scientists to find a control group that hasn't used mobile phones.