I have diabetes. This is an excellent question, and one that we as educators get asked frequently. I made a call to the Canadian Blood Services, and can tell you the following.
Timing is also of great importance.
Here are suggested waiting times following an activity or ailment. Refusing a blood donation from someone who has diabetes and is on insulin is for the donor's benefit. Such a donation could interrupt your balance or glucose control. In addition, there would be concern about having to treat a low blood sugar reaction, something which could occur on site or, more problematically, on the way home. Those who control their diabetes through diet, exercise or oral diabetes pills are able to donate under most circumstances.