divots/grooves in teeth

For the last week I've felt like there's something in my top lateral incisors, the side that is facing in my mouth.  I felt it with my tongue at first, and then with my finger, trying to pick it off.  No amount of flossing got rid of it, and I have become more and more conscious of the sensation of something "in my teeth".

Finally, I took a picture to see what's going on, and there is a pretty deep.. "groove" in my teeth.  The affected teeth also look a little spongy.  It seems that this phenomenon is symmetrical (I only have pictures of one side).  It's rather sharp, but not sensitive.  I can push the insides with my fingernail and only feel pressure.

See here:
http://img301.imageshack.us/img301/4280/img3679vq9.jpg
http://img182.imageshack.us/img182/8861/img3681ru9.jpg

What exactly is going on here?  Should I go to a dentist?