Xorg has been recently updated to 7.3 and apparently uses xrandr to set the best resolution for you.
It works well in most cases, but sometimes, you'll want to set X to start in a resolution YOU choose.
Of course, the first thing you do is to tweak your xorg.conf as you did it for about x years now and happily press [CTRL][ALT][<--] and ..? wtf o_O the resolution has not changed. After having checked twice your xorg.conf, you're still stucked in that resolution and your desktop is unusable due to all icons in your gnome-panel that are not visible anymore... So, how to modify xorg.conf to your needs? Here is it! Say you want a resolution set to 1600x1200 at 75Hz, you use gtf tool to output a Modeline that'll be used in xorg later:
$ gtf 1600 1200 75