How to find cursor position in a contenteditable DIV?

If all you want to do is insert some content at the cursor, there's no need to find its position explicitly. The following function will insert a DOM node (element or text node) at the cursor position in all the mainstream desktop browsers:

function insertNodeAtCursor(node) {
    var range, html;
    if (window.getSelection && window.getSelection().getRangeAt) {
        range = window.getSelection().getRangeAt(0);
        range.insertNode(node);
    } else if (document.selection && document.selection.createRange) {
        range = document.selection.createRange();
        html = (node.nodeType == 3) ? node.data : node.outerHTML;
        range.pasteHTML(html);
    }
}

If you would rather insert an HTML string:

function insertHtmlAtCursor(html) {
    var range, node;
    if (window.getSelection && window.getSelection().getRangeAt) {
        range = window.getSelection().getRangeAt(0);
        node = range.createContextualFragment(html);
        range.insertNode(node);
    } else if (document.selection && document.selection.createRange) {
        document.selection.createRange().pasteHTML(html);
    }
}

UPDATE

Following the OP's comments, I suggest using my own Rangy library, which adds a wrapper to IE TextRange object that behaves like a DOM Range. A DOM Range consists of a start and end boundary, each of which is expressed in terms of a node and an offset within that node, and a bunch of methods for manipulating the Range. The MDC article should provide some introduction.