At a routine cleaning and check-up my dentist noted some probable decay in the rear-most molar on the bottom left. He also mentioned that the size, shape, and expansion characteristics of the filling probably meant it acted as a wedge/maul and over time created a small crack in the tooth. At the time of this writing it wasn't clear whether the crack existed.
He recommended removing the old filling and repairing/restoring it with a crown. There was some "hopefully you won't need a root canal" discussion as well. The dentist said it would take about 2 hours, assuming all goes smoothly..."no big deal, it is a good movie" (he has an entertainment system)
All things considered, the whole situation stinks. It promises to be somewhat painful and expensive. I am not looking forward to it at all and have been complaining to everyone I know. It isn't the dentist, it is the situation.
I'll let you all know how it goes.