I had one. I had to go into an "emergency" visit to the dentist over a weekend to get it checked out. It looked like I had a canker sore on my gum, but it turned out my tooth was absessing (sp?) out of my gum. . .which is really pretty gross.
The kicker was my whole jaw was hurting, and it turned out, that my absessed tooth started an infection in my jaw and ate away quite a bit of bone, hence, the constant aches I was feeling in my jaw. So, had to have oral surgery to remove the infection, put on antibiotics, the bone was able to heal itself.
This stemmed from a previous injury of me running into a flagpole, in which one of my teeth died without me knowing, and that was pretty much how the story started.
I also disagree with moshquerade. If the tooth has died, it wont necessarily ache. I never had a toothache, only pains in my jaw.