Alex, over at Sweat Science covered some research that was announced back in August at the European Society of Cardiology (press release and abstract here). He took issue with some of the reported findings and I largely agree with his assessment. These results were presented at a scientific meeting, not published in a peer-reviewed journal, and therefore should be considered preliminary.
The authors included all arrhythmias in their research, but some of those are expected results of training, like bradycardia, while others, like atrial fibrillation, are dangerous. The abstract provides data on a couple of subtypes of arrhythmias, but not atrial fibrillation so we do not really what role it played in the outcomes. Atrial fibrillation (AFib) is a particular concern for active people because it can cause serious problems with your heart rate and some of the medicines used for it can make it harder for you to get a good athletic performance. Also, many people with AFib have to be on blood thinners like warfarin to reduce the risk of stroke, but blood thinners can be dangerous if you then get in an accident.
Hopefully some of these questions will be addressed in a full journal manuscript. As Alex points out, even if risk of AFib is increased, regular exercise has shown time and time again to extend life, so I will keep on running.