It can vary by material, but the printing processes at the large Service Bureaus often free you from the concern of overhangs. The industrial Selective Laser Sintering (SLS) printers used for Nylon pieces, for example, have an ingenious way to get around the gravity problem. The whole print bed is covered in a layer of nylon powder. A laser will heat up and solidify just the sections of powder that will become the first layer of your object. A fresh layer of powder is laid down on the printer and the laser will solidify the next layer of your object. The printing process finishes with a big block of powder on the printer. Your piece is dug out of the loose powder, much like an archaeologist excavating pottery from soil.
Because the excess powder provides a natural support system, even completely horizontal overhangs are achievable. You also don't have to worry about flat bases and having solid contact with the print bed. The extra powder from previous layers is there to act as a foundation no matter where or how small your object starts.