In the recent past, I used to switch screen resolutions in XFree/X.org by pressing the CTRL-ALT-minus or CTRL-ALT-plus keys. For each supported mode, there used to be a ModeLine entry in xorg.conf and those entries were referenced by the "Screen" section. As often happens when technology "advances", on my new Dell D830 notebook (using xorg 1:7.3+18 from Debian 'testing', GM965/GM960 graphics controller, 'intel' display driver), this tried and true method no longer works. The magic key combo is ignored altogether, as is adding ModeLines to xorg.conf. No warnings in /var/log/Xorg.0.log nor anywhere else are recorded, AFAICS. The standard xorg.conf seems almost empty, which in itself is a good thing (less configuration means less opportunity for mistakes). The bad thing, obviously, is that invalid or deprecated (?) configuration entries are silently skipped without a clue for the user about what is going on inside the black box, whether its behavior is normal or not, and which alternatives might be available. This is an example of what I like to call an "observability" problem. Sometimes a careless implementation is to blame, but such problems may also have their roots in software designers' inability to correctly foresee and specify the possible variance in implementations.
Fortunately, there is an easy solution to ignored ModeLines. xorg includes a program called
xrandr, which can be used to test and programmatically set a resolution, as described in the Ubuntu Wiki article X/Config/Resolution. This seems to be a substitute for the traditional ModeLines in xorg.conf. Although the Wiki article mentions that xorg.conf can still be used for specifying resolutions, this is simply not the case in my setup.