The American Heart Association has released new guidelines regarding prevention of endocarditis during dental and other procedures. Essentially, almost no one needs it anymore. See the details here.
For those unaware, there has long been a concern that the introduction of bacteria to the blood stream during dental cleaning could lead to infection of the heart, leading to disastrous consequences. People thought to be at higher risk included those with prosthetic valves, congenital heart defects, and the much more common "leaky valve". It has long been thought that the risk of infection was quite overblown, but due to liability concerns you'd NEVER find a dentist to touch your teeth without premedication with an antibiotic, if you had any history suggesting a valvular problem. Hopefully that will all be able to change now.
Fewer unneccessary antibiotics is always a good thing.