In countries like Africa where AIDS is a huge problem, the root problem is people having unprotected sex. While your point is valid for developed countries, I don't think it can apply to where the disease is at its worse, which is what I was refering to - although I should have pointed that out when I posted.
I'll probably get bashed pretty hard for saying this, as it could be percieved as, like Hali said, immoral (which it probably is, but its realistic none-the-less), but diseases like HIV/AIDS in countries such as Africa actually play an important role. By giving them a vaccination, it won't stop them from having unprotected sex and they'll continue to have children at unsustainable rate. It might prevent AIDS, but in the long run it'll cause bigger problems.
In my opinion, the best prevention for diseases for such as AIDS is by educating people of the dangers of unprotected sex, and doing as much as humanly possible to influence people in countries where HIV/AIDS is a big problem to wear condoms and other forms of contraception.
To wrap my point up, I think a HIV/AIDS vaccine would be a great thing, but I think too many people would see it as a license to go out and have unprotected sex. Alot of people would see it as a form of contraception, which it isn't - it's a vaccine.