edit: I was in the midst of Adobe AIR for Android development when I answered this question, and looking back at it, I realize this question didn't pertain to Adobe AIR.
Adobe says:
On devices that can change the screen orientation, such as mobile phones, a Video object attached to the camera will only show upright video in a landscape-aspect orientation. Thus, mobile apps should use a landscape orientation when displaying video and should not auto-rotate.
http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/media/Camera.html
If you do really want to use the camera in portrait mode, my suggestion is to rotate the video object.
Here's some sample code that rotates the video object (_video) by an angle in degrees (source was pulled from elsewhere on stackoverflow):
var matrix:Matrix = _video.transform.matrix;
var rect:Rectangle = _video.getBounds(this);
matrix.translate(- (rect.left + (rect.width/2)), - (rect.top + (rect.height/2)));
matrix.rotate((angle/180)*Math.PI);
matrix.translate(rect.left + (rect.width/2), rect.top + (rect.height/2));
_video.transform.matrix = matrix;