I picked this off the net,it is about Canada but it probably applies world wide.
Here are some quotes from the Canadian Diabetes Association web page
"If you take insulin, you are not eligible to donate. The rationale is that donating blood could interrupt your blood glucose control, and potentially lead to a hypoglycemic reaction in the clinic or on the way home.
If you manage your diabetes with lifestyle or diabetes pills, you may be eligible, depending on your overall health and if you meet the requirements listed below. Many people who have type 2 diabetes have blood pressure and heart problems, which would prevent them from donating.
Canadian Blood Services divides willing donors into 3 categories: those who can donate right away, those who can donate after a waiting period and those who cannot donate. Some criteria for each category are listed below. These rules are in place to protect both the donor and to ensure the safety of the Canadian blood supply."
"Donations may not be taken from you if you:
* Are taking insulin.
* Have certain diseases or medical conditions. Check with your local clinic.
* Have lived in certain regions of Africa, where you may have been exposed to a new strain of the virus that causes AIDS (HIV-I Group O), or if you received a blood transfusion while visiting there, or have had sex with someone who has lived there. This is based not on race or ethnicity, but rather possible exposure to HIV-I Group O. Countries included are: Cameroon, Central African Republic, Chad, Congo, Equatorial Guinea, Gabon, Niger and Nigeria.
* May have been exposed to HIV, including sexual activity with someone who has tested positive for HIV.
* Had possible exposure to CJD (Creutzfeldt Jakob Disease) or vCJD (Variant Creutzfeldt Jakob Disease) between specific dates and in specific geographic areas (check with your local clinic)."
and here is a quote off of the Canadian blood services page.
"5. Diabetes:
If your diabetes is treated with insulin, you are unable to donate blood."
short and sweet ... if you are diabetic and treat with Insulin ... NO.