HIV and AIDS have only been around for 20-30 years. So while HIV can cause AIDS and will for the vast majority infected. However it is not yet known for certain whether it will period as there has been more than one case of HIV dissapearing from an infected person.
Actually, HIV was first discovered in 1959. Most believe it has been in the human population since the mid 1940s (probably from humans eating primates that were infected with the SIV virus which mutated). I believe the first case in the US was identified in 1969.
It is true that a couple of people have had it disappear, but there is by no means any evidence to suggest that it won't eventually lead to AIDS. With the exception of those few rare cases, everyone has either gotten AIDS or died before the had the chance to.
Remember, AIDS is not a disease but a syndrome. When the immune system drops below a certain strength you have AIDS. HIV attacks T-helper cells (which are part of the immune system) and therefore weaken it. Once it manages to weaken it far enough-the person is said to have AIDS. Medication coctails can help keep the immune system strong and to fight but in the end it is a losing battle. But, patients who are given the proper medication can sometimes live for 20 years or more with HIV before AIDS sets in.
Also, AIDS doesn't kill you because you can conceviably live without an immune system. What kills you is a secondary infection. Because your immune system is so weak, it is easy to get other illnesses. If I remember right, the majority of people who die as a result of AIDS are actually killed by pneumonia.