Could a cavity in a human tooth "heal over" or possibly fill back in?
The answer is a simple no. Cavities in the teeth caused by dental decay cannot heal. Enamel which is affected by dental decay cannot heal as they are fully formed at the time of eruption and then on only deteriorates. The deeper part of the tooth which has dentin cannot heal the cavity formation, however it can produce a protective barrier between the advancing cavity front and the pulp inside. The cementum the third mineralised tissue of the tooth cannot heal a decay cavity. Its main function is to attach the tooth to the bony socket through the periodontal fibres.
Yes, it can be done to a certain degree.
Cementum is capable of repairing itself to a limited degree and is not resorbed under normal conditions. Some root resorption of the apical portion of the root may occur, however, if orthodontic pressures are excessive and movement is too fast. Some experts also agree on a third type of cementum, afibrillar cementum, which sometimes extends onto the enamel of the tooth.
Cementum is produced by cells called cementoblasts.
more here: http://en.wikipedia.org/wiki/Cementum