The NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases) has an informative site that summarizes the how-to's of exercise when you have diabetes.
It describes different types of exercises (aerobics, strength training, stretching), what exercises to avoid, and what to do if your blood sugar dips too low (a condition known as hypoglycemia).
"Hypoglycemia can happen while you exercise, right afterward, or even up to a day later. It can make you feel shaky, weak, confused, irritable, hungry, or tired. You may sweat a lot or get a headache. If your blood glucose drops too low, you could pass out or have a seizure."
It's a good idea to check your blood sugar during and after any extended period of activity. For more on exercising with diabetes visit the NIDDK's What I Need to Know About Physical Activity and Diabetes