A paper only published 3 days ago (Feb 5, 2008) showed that antiretroviral drugs may protect from HIV transmission. Macaque monkeys who were all infected with SHIV (a virus close to HIV) anally were less likely to get infected if they were taking antiretroviral drugs. Pre-exposure Prophylaxis treatment has been found to be effective in Malaria, but it has not been proven to be effective in a virus relating to HIV until now. To simulate sexual transmission of HIV, researchers gave the virus to the monkeys at their anus.
The article and references can be found here: