Really, it's rather a small problem and I wouldn't say it's going to end my life any time soon, but I could have sworn I was able to use 1920x1080 as a windowed mode resolution in the past.
Regardless of whether or not this is true I would very much like to see the option, if possible! I tried editing the config.ini file, which set the window size upon launch, only to be set back to 1600x1200 a split second later and subsequently showing off all the different logos during startup.
In short: would I possibly be able to see the 1920x1080 window mode resolution (again?) any time soon?
Defense Grid gets "valid" resolutions from Windows. Windows may now be telling the game different resolutions are valid than used to be valid due to either an updated or changed video driver or a changed understanding of what hardware monitor is present. Check the monitor setting in your Windows device manager and see if it thinks you might have a monitor that has less resolution than before, or check to see if you have the latest video driver for your video card. That likely will fix the problem.