I have noticed the black spots/stripes as well. It is the invisible Shield by ZAGG. I have replaced my shield on my galaxy S2 already and I have only had the phone for 2 months. My Xoom also has a stripe, but i have had it for 8 months, and haven't replaced it yet.
I don't know what causes the stripe, but ZAGG is useless to try and talk to. It is something to do with the device. The black spot appeared in the same spot both times.
I think i am going to try something other than Invisible shield.