I would like my game to be playable only in landscape mode. However, I would obviously like it to be cross compatible.
Personally I would just use agk::SetOrientationAllowed(0,0,1,1) to force landscape mode, then choose a resolution I'm happy with, say agk::SetVirtualResolution(500,350), and then code the game assuming that screen size.
For iOS and Bada the game will always be drawn in landscape mode (since we specified landscape only) and on desktop platforms use setup.agc to create a landscape shaped window in whatever size you like. AppGameKit will fill it as much as possible with the 500x350 game.