In IE >= 9 and non-IE browsers (Firefox 4+, WebKit browsers released since early 2009, Opera 11, maybe earlier), you can use the getClientRects() method of Range. In IE 4 - 10, you can use the boundingLeft and boundingTop properties of the TextRange that can be extracted from the selection. Here's a function that will do what you want in recent browsers.

Note that there are some situations in which you may wrongly get co-ordinates 0, 0, as mentioned in the comments by @Louis. In that case you'll have to fall back to a workaround of temporarily inserting an element and getting its position.

jsFiddle: http://jsfiddle.net/NFJ9r/132/

Code:

function getSelectionCoords(win) {

    win = win || window;

    var doc = win.document;

    var sel = doc.selection, range, rects, rect;

    var x = 0, y = 0;

    if (sel) {

        if (sel.type != Control) {

            range = sel.createRange();

            range.collapse(true);

            x = range.boundingLeft;

            y = range.boundingTop;

        }

    } else if (win.getSelection) {

        sel = win.getSelection();

        if (sel.rangeCount) {

            range = sel.getRangeAt(0).cloneRange();

            if (range.getClientRects) {

                range.collapse(true);

                rects = range.getClientRects();

                if (rects.length > 0) {

                    rect = rects[0];

                }

                x = rect.left;

                y = rect.top;

            }

            // Fall back to inserting a temporary element

            if (x == 0 && y == 0) {

                var span = doc.createElement(span);

                if (span.getClientRects) {

                    // Ensure span has dimensions and position by

                    // adding a zero-width space character

                    span.appendChild( doc.createTextNode(u200b) );

                    range.insertNode(span);

                    rect = span.getClientRects()[0];

                    x = rect.left;

                    y = rect.top;

                    var spanParent = span.parentNode;

                    spanParent.removeChild(span);



                    // Glue any broken text nodes back together

                    spanParent.normalize();

                }

            }

        }

    }

    return { x: x, y: y };

}

UPDATE

I submitted a WebKit bug as a result of the comments, and it's now been fixed.

https://bugs.webkit.org/show_bug.cgi?id=65324