Mine too... in fact I find it annoying as I frequently find myself moving my thumb over the sensor and making the display go dim while I'm still in bright light. I have to be careful how I hold the device so the display stays bright enough to be comfortable.
I even noticed it today at work when I walked from a brightly lit part of a hallway to a slightly less well lit part - and my display got considerably dimmer. I had to tilt it towards the light to get the display to brighten back up.

Don't all of 'em do this? (running 1.3 firmware)