I only have personal experiences and discussions/panels I've done with professional fitness/weighlifters that have diabetes:
Working out affects diabetes in many ways, but the two main ones are the differences between aerobic and anaerobic exercises. Workouts like running (aerobic) will lower the blood sugar because it uses the glucose in the blood to continuously feed the cells. The opposite of that is exercises like weightlifting, especially days when you do the big, major muscle groups. This is caused, in part, by 'squeezing' the glucose out of the muscles during lifting exercises. The one problem this causes is when someone sees a major post-lifing high, the first reaction is to correct that high with insulin. Anywhere from 2 - 12 hours after a high intensity workout that will come back to bite you as your body readjusts and the blood sugar will drop again. One thing that is definately a must, as Pat pointed out: if you are not using a pump, and are using a long acting insulin such as Lantus, you must cut your dose pre-workout. As for pre-workout eating it varies for everyone. I was playing football in college when I was diagnosed and it took me a long time to get it down just right, but I figured out that if I drank a gatorade and ate a nutrigrain bar, I was good to go for a 3-4 hour workout. However, any diabetic, especially anyone new and just figuring things out, must keep some type of sugar near by during intense workouts as a safety precaution. A dramatic low can hit you so fast you might not make it to the lockeroom or your car.
I hope this helps. Any other questions just shoot them over to me and I'll help any way I can.