OK, I know you guys down voted paolap, however, for whatever reason, that worked for me. When I connect by 1440 x 900 monitor to my macbook pro, it tends to default to a 800 x 600 resolution, and I am not able to change it (or at least change it to what I wanted, even with the use of the alt key). It seems that somehow the order and timing of how things are plugged in and turned on can "magically" cause it to occasionally (1/5 times) default to the correct resolution. My theory is that somewhere, the monitor tries to tell the computer the correct resolution, but if the computer already decides to send a 800 x 600 signal, the monitor just decides to go with it. So somehow unplugging and replugging can sometimes make the computer realize what the correct resolution should be.