Yeah, it's been kicking up in popularity lately. Either rotating your monitor and running games in portrait mode, or straight up forcing a portrait resolution with pillarboxing (highest I can go is 2400x3200 for 3:4).
ninja edit: added another
Really like that second one.
A quick note that I'll probly also add to the compendium:
I was having a hard time getting portrait res's of any high value to work on my monitor. It would stubbornly refuse even though the total pixel count would be below my max (~9mil at 2400p).
But then, as a test, I lowered the active pixels horizontal value, thinking maybe that it should reflect the actual AR (i.e it said 1920x1200, i made it 960x1200). This produced a different result, but it didn't work until I used 1440x1200.
So the point is, if you want to try and crank up your portrait res (one that doesnt rotate with your screen), try lowering the active pixels value. The problem is I have no guideline for this. Your mileage may vary. I wonder what the overall implications of that is...