Is "1920x1080" minimum resolution really the minimum? My current monitor supports a maximum of 1440 x 900.
Yes, 1920x1080 is the min spec. for resolution.
May I make a very very very serious recommendation
Ensure you put that in GIGANTIC NEON ORANGE BOX on the Steam Store page
Seriously. A guy in one forum literally went ape stupid on a dev because the game had a minimum res of 1366 x 768 and he was trying to run it on his 1024x768 plasma display. Basically "This is a travesty I dont want to buy a new tv blah blha waaahhh"
If you don't make that ultra super clear on the Store page you can bet the Steam community will lose their collective skulls over it. They did for DX11 games and for games taht require 64-bit. I am quite confident they will continue their downward trend of not reading stuff.
Though I should ask, is this just for the alpha or the intention to carry the 1080p resolution requirement to full release.