I've spoken to chief a couple times about this, and he seems to agree it's a good idea..
Basically, for those who don't know what it is from the title, fullscreen window-mode, is basically window-mode without a frame, behaviourally the client would act like it's in window mode for alt-tab purposes, but it wouldn't be moveable, nor resizeable (*locked to screen resolution), whilst not too much of a concern for anything debugging-ish if you have a 20+in monitor, if you're stuck on lower resolutions especially, it can seriously help debugging/minor bug reporting (because you can alt-tab to fred or notepad without losing resolution).
At the moment the closest approximation is moving the window bar as far off screen (up) as you can and lining the sides up with the monitor but this doesn't over-ride the task bar and feels very tacky in comparison to a proper full screen window mode.
I was wondering if anyone might have any idea about implementing this as, as I said; it can be a real space saver, and really useful on smaller monitors, it also makes the alt-tab experience more stable because, as with window mode, the game is still being fully rendered in the background (although paused or what have you) which means there's no massive framerate drop when you tab back to it whilst the pc catches up on the cycles it lost (not sure what causes that either).
Honestly, it would be massively, massively useful for me, dunno about anyone else, but especially with games I'm likely to spend a long session on, fullscreen window mode is almost a must.