I was told that it wasn't the leads I needed to worry about, but the MRI would mess up the settings of my pacemaker. Maybe different answers from different people. The next time I see the guy on my mail route that works for St Jude, I'll ask him.
Ask your cardiologist about the specifics of what your pacemaker would sense, and how it would react in different situations. Take into account what would normally happen if your heart was at the end or start of an event that would normally cause in increase or decrease in heart rate, finishing a run or ride, etc.