Bit sharpening services are available through Woodcraft. You can also check your local saw sharpening services or tool & die shops, some offer bit sharpening. Here is the situation: a dull bit may have the face cleaned to provide a new sharp edge. This will not significantly alter the bits diameter. Remember we are talking a matter of perhaps .002". The trick is to be able to accurately remove an identical amount from both cutting surfaces, and that is not something you can do at home. While most bits can be sharpened a couple times quality bits such as Amana, CMT or Whiteside will withstand more sharpenings before losing tolerance. Any bit with a chip in the edge is done. When it comes to $10 bits you are better off replaceing than paying perhaps $8 to have them sharpened, but a $60 plunge/roundover bit from one of "the big three" is a sound investment.