I've just moved back from Australia where for the last 3 years I have been able to give blood, despite being T1. As long as your doctor signs off that you have generally good control, you are able to donate. I believe you can generally donate in the USA also.
I did call the UK Blood Service and ask for the reasons why they don't accept T1's and all they could tell me was that it was the current recommendation from their medical advisors. Not much of an answer!