If you're adjusting your television by eye or using the default settings, you may not notice this issue simply because your television is likely set incorrectly based on existing video standards. And like the vast majority of consumers, you may love the way it looks... but wrong is still wrong. Movies are created using calibrated monitors to a set standard, and for you to properly display what the director and cinematographer intended you to see, your display has to be set to those same standards.
If you have a PS3 or Blu-ray player, use the test patterns linked above to adjust your display so that everything below 16 is one solid black with no flashing bars but with the bar at 17 barely visible. It's okay to leave white above 235 unless you're trying to maximize the contrast ratio of your display, though some Blu-ray discs have white highlights above 235. The danger of pushing white level to clipping at 235 is that you may also clip the color channels (which extend above 235). My Epson 8500UB (which accepts both PAL and NTSC so I can play PAL DVDs with my Oppo BDP-83) is calibrated using an EyeOne Display LT colorimeter so that the color channels don't clip until above 240 and white peaks right at 255, since I have contrast to spare. After you've correctly calibrated your set's black and white level, go back to your 360 and play the same test pattern from AVS. If things were working correctly on the 360, it would match up to your other sources (i.e. black at 16). With the new dashboard, it doesn't... though it did on the previous dashboard.
For this device to remain compliant with existing video standards, Standard reference level MUST comply with the 16-235 video levels for television output and Extended must comply with the 0-255 standard (for computer monitors). If it doesn't, then it won't match your other video sources (cable/satellite, Blu-ray, other game consoles, etc.). Therefore, this IS a system-wide issue across all regions.