Making a rotated gradient in Actionscript is not a trivial task. It turns out there’s only one method to do it that works correctly for me (all other Matrix type voodoo produce funky results). This did exactly what I wanted though:

var matrix:Matrix = new Matrix();

// This line of code is the magic ingredient:
matrix.createGradientBox(_h, _w, (0.5*Math.PI), 0, 0);

with(_bg.graphics) {
clear();
beginGradientFill("linear", [0x656565,0x323232], [100,100], [0,255], matrix);
drawRect(0,0,_w,_h);
endFill();
}

In the createGradientBox, I am doing two things. First I’m creating a box with the precise dimensions of my final gradient, except pre-rotated (flipping width and height) so that when the gradient draws itself out, it does so in its normal way (horizontally across). Secondly, I’m rotating it 90 degrees (or 1/2 PI in radians), so that the width and height flip back to normal as a result of the rotation. What this also accomplishes, however, is it rotates the horizontally rendered gradient to be vertical.