Get the scale value of an element?

The simplest solution to find out the scale factor between the document and an element is the following:

var element = document.querySelector('...');
var scaleX = element.getBoundingClientRect().width / element.offsetWidth;

This works because getBoundingClientRect returns the actual dimension while offsetWidth/Height is the unscaled size.

If it was specified by a matrix I guess you can't with a straightforward way, but you can easily parse the value:

var matrixRegex = /matrix\((-?\d*\.?\d+),\s*0,\s*0,\s*(-?\d*\.?\d+),\s*0,\s*0\)/,
    matches = $(element).css('-webkit-transform').match(matrixRegex);

matches[1] will contain scaleX and matches[2] will contain scaleY. If it's possible that other transformations have also been applied, you'd need to slightly tweak the regex, because now it assumes that all other parameters are 0.

A way to just get the scale values might be to remove any transforms, measure the computed width/height of the element and then add them back and measure again. Then divide new/old values. Haven't tried it, but it might work. jQuery itself uses a similar approach for many measurements, it even has an undocumented $.swap() function just for this.

PS: You are using -o-transform -moz-transform and -ms-transform too, right?

If you need to target webkit only (because it's for the iPhone, or iPad) the most reliable and fast way is using the native javascript webkit provides:

node = $("#yourid")[0];
var curTransform = new WebKitCSSMatrix(window.getComputedStyle(node).webkitTransform);
alert(curTransform.a); // curTransform is an object,
alert(curTransform.d); // a through f represent all values of the transformation matrix

You can view a demo here: http://jsfiddle.net/umZHA/

You could use the following:

var element = document.getElementById("elementID");
// returns matrix(1,0,0,1,0,0)
var matrix = window.getComputedStyle(element).transform;
var matrixArray = matrix.replace("matrix(", "").split(",");
var scaleX = parseFloat(matrixArray[0]); // convert from string to number
var scaleY = parseFloat(matrixArray[3]);
// bonus round - gets translate values
var translateX = parseFloat(matrixArray[4]); 
var translateY = parseFloat(matrixArray[5]); // parseFloat ignores ")"