There are many display devices - LCD panels, plasmas, LCD and DLP projectors that will not 1:1 pixel map even if you feed the display it's native resolution.
Many display devices will over-scale even its native resolution in order to overscan the display borders.
To verify if you are really 1:1 mapping, you need to feed your display a 1 pixel pattern such as:
http://www.marky.com/files/hometheater/testpatterns/tracking-and-phase.gif
and verify there is no noise, moire patterns, or periodic light/dark vertical bands in the tiled test pattern. Simply tile the test pattern on any Windows or Linux desktop, i.e. unscaled by the PC, then set the desktop resolution to the native res of your display at 60Hz.
One recent example that caused quite an uproar in the digital projectors forum is the Sony HS51 projector.