I bought my Kindle Keyboard in August of 2010. Right when it came out. It got that crack on the right bottom bezel about 6-7 months into ownership. I watched it for a while and it didn't get longer. But it bugged me to look at it.
I didn't want to go through the trouble with exchange.
So I got a matte black wood skin from decal girl and stuck it on. I wanted something t be as close to the original color as possible as I was afraid, I wouldn't be able to stick it on straight. Then
that would have bugged me.
My KK has been fine every since. Not a problem. I don't think the crack got any bigger, but I don't know for sure as I can't see it anymore.
I do know it did not extent to the edge as there is no skin on there and I can't see it.
So more than 2 years with the "crack" and no issues.