HIV is a very, very small virus, it measures about four millionths of an inch across, or one twentieth of the length of an E. Coli bacterium. HIV is a retrovirus, meaning its genetic information is stored in RNA. The virus only has nine genes, three of which govern the production of new virus structures, and six of which control the infection of cells and the effects on human cells.
When HIV encounters human T cells, carries of the CD4 surface protein, they meld with the cell membrane and enter the cell, leaving their own membranes behind. Now comes the scary part. The virus then transfers its RNA to DNA which is then combined into the genetic material of the nucleus. The human cells will help the virus replicate itself, so it can go out and create more "buddies" this is one of the reasons that the virus is so hard to treat or rid from the body.
Because of the nature of the virus and its mechanisms, it is not surprising that no cure has been found. For example, the virus can only be seen with an electron microscope, relatively new technology. With medicine advancing at a fast rate, it is not out of the realm of possibility that a cure will be found, even in the next 20 years or so. As we learn more about the body, natural systems and disease mechanisms, and the knowledge of these areas becomes intermingled, allowing greater understanding that will hopefully lead to a cure, or perhaps even a treatment to counteract the actions of the virus.