I am a runner, and I had this EXACT question at first. According to my OB, running is OK as long as you keep your heart rate below 140. I wasn't able to keep it below that level, so she told me I had to stop running and start doing another exercise. I now use the elliptical machine at the gym. The reason you need to keep the heart rate under 140 is because the baby's heart rate is proportionately tied to yours, so raising your heart rate too much will make the baby's race off the charts. Good luck!
Runner on pause...8 weeks pregnant with blessing #1...asked the OB this question