It's flat, SuperShieldz sells some nice glass screen protectors that fit well on Amazon pretty cheap. Just search for the ones by them that specifically say they're compatible with the 2X.
It doesn't appear to have any effect for the glass ones, the lux hours meter still seems to tick up by the same amount every time it refreshes when in direct sunlight. I was concerned about that as well so I tested it before and after on the same sunny day.
