I haven't personally heard anything about Hiv tainted blood getting into the blood supply since 9/11 but I have heard stories of people going to donate blood for the first time and later after their blood was tested finding out they had Hiv.This makes much more sense with what I have seen when donating blood.
Red Cross requires you to agree to have your blood tested for Hiv and hepititis before you donate here. Then you fill out a form and answer questions that feret out if you are at risk, then you put a sticker on the form in private that tells them if you want your blood used or not just in case you were too imbarrased to tell the truth on the form ,then your blood is tested anyway no matter what your answer is.
Also I have found this interesting,They informed me in the literature that I had to read ahead of time that my blood would be seperated into three components if found useable.The Red cross may do it differently in other areas but this is the way it is here.
Ranchette