function inchesToPixels(inches:Number):uint
{
   return Math.round(Capabilities.screenDPI * inches);
}


var myWidthInInches:Number = 1.5;
var myWidthInPixels:uint = inchesToPixels(myWidthInInches);
trace("myWidthInPixels: "+myWidthInPixels);

// OUTPUT
// myWidthInPixels: 108

http://www.adobe.com/devnet/flash/articles/authoring_for_multiple_screen_sizes.html

This function is from Christian Cantrell's Adobe article called 'Authoring mobile Flash content for multiple screen sizes'.

AS3 Inches to Pixels

ActionScript 3