“It's not enough to ask a prospective sexual partner if they are HIV-infected. The reality is that someone can be telling the truth based on a negative test report received that same morning, which only means they tested negative for HIV antibodies two weeks ago, when the blood for the test was drawn. It may even be that they carried HIV in their bloodstream but had not yet produced HIV antibodies.” –Old tactics, old tests can't stop modern plague, by Dr. Mark Wainberg (director of the McGill University AIDS Centre)
I have no idea how many people are even posing the question or getting tested (rising HIV transmission rates would lead me to believe that not enough are doing either) but a recent Quebec study showed that up to 50% of new HIV cases in the province were spread by people who had been HIV-positive for less than six months. So by all means we should ask about a sexual partner's HIV and STD status but relying on his/her answer is enormously risky.
Check out Scarleteen's guide on when to use what (and other safety essentials):
• Safe, Sound & Sexy: A Safer Sex How-To