Although details so far are sparse, a recent study from the University of Oklahoma appears to indicate that vitamin C may help to prevent vascular damage in people with type 1 diabetes. A followup study will be done to see whether similar benefits are found for type 2.

A large proportion of the available reports so far include a statement that the combination of insulin and vitamin C reduces complications in people with type 1 and "poor glucose control", which makes me wonder how much benefit vitamin C will have for those who already have good control, but previous studies have shown the vitamin's antioxidant effects to be helpful, particularly for people with diabetes, but also in the general population.

It's also unclear how well these findings will carry over to normal dietary intake of vitamin C, as

but oranges are tasty and good for you even if they don't specifically help with diabetes. So enjoy one!

Links:
OU researchers stop diabetes damage with Vitamin C
Vitamin C Helps Diabetes Patients
Vitamin C Helps Reduce Complications Linked to Type 1 Diabetes