Good idea.  There's a few minor niggles, though:

1. Don't use `unsafeWindow`, this will break in Chrome and will not even work in Firefox+Greasemonkey if the effective `@grant` changes.

1. Avoid leading `*` wildcards in `@includes`, if you can help it.  This is a big performance hit.  `@match` limits the wildcard scope and performs much better and reports the sites/security correctly on Chrome.  This gets all current SE sites:

        // @match       http://*.askubuntu.com/questions/*
        // @match       http://*.onstartups.com/questions/*
        // @match       http://*.serverfault.com/questions/*
        // @match       http://*.stackapps.com/questions/*
        // @match       http://*.stackexchange.com/questions/*
        // @match       http://*.stackoverflow.com/questions/*
        // @match       http://*.superuser.com/questions/*

1. The regex `match` needs to be more robust, to account for all legal characters in the URL, and for anchors.

1. Use injection so that the script will work on most userscript-capable browsers (Chrome, Firefox, Opera, Safari, etc.).

1. Mathoverflow.net is not an SE site!  Use [math.stackexchange.com][1] for that.  mathoverflow.net also does not appear to support the *timeline* view/page.

<br>
Here's a variant of the script I've been using (It works on Chrome and Firefox, and should work on most other userscript-capable browsers):

[**Easy install URL at GitHub**][2] for Greasemonkey, Tampermonkey, Scriptish, etc.

    // ==UserScript==
    // @name        Add Timeline Button to Stack Exchange questions
    // @description Adds a "View Timeline" Button to Stack Exchange sites
    // @namespace   StackExchange
    // @match       http://*.askubuntu.com/questions/*
    // @match       http://*.onstartups.com/questions/*
    // @match       http://*.serverfault.com/questions/*
    // @match       http://*.stackapps.com/questions/*
    // @match       http://*.stackexchange.com/questions/*
    // @match       http://*.stackoverflow.com/questions/*
    // @match       http://*.superuser.com/questions/*
    // ==/UserScript==

    function addTimelineButton ($) {
        var buttonText      = "View Timeline";
        var timelineLink    = "#";
        var postNum         = 0;

        var postMatch   = location.pathname.match (/questions\/(\d+)\D/i);
        if (postMatch  &&  postMatch.length > 1) {
            postNum         = postMatch[1];
            timelineLink    = location.protocol + "//"
                            + location.host
                            + "/posts/"
                            + postNum + "/timeline"
                            ;
        }
        else {
            buttonText  = "Path error 1";
        }

        var targetNode  = $(".favoritecount");
        if (targetNode.length) {
            targetNode.after ('<a href="' + timelineLink + '">' + buttonText + '</a>');
        }
        else if (
            postNum
            &&  $("#content div.subheader > h1:contains('Page Not Found')").length == 0
        ) {
            alert ("From Timeline script:  Unexpected Page layout.");
        }
    }

    withPages_jQuery (addTimelineButton);

    function withPages_jQuery (NAMED_FunctionToRun) {
        //--- Use named functions for clarity and debugging...
        var funcText        = NAMED_FunctionToRun.toString ();
        var funcName        = funcText.replace (/^function\s+(\w+)\s*\((.|\n|\r)+$/, "$1");
        var script          = document.createElement ("script");
        script.textContent  = funcText + "\n\n";
        script.textContent += 'jQuery(document).ready( function () {' + funcName + '(jQuery);} );';
        document.body.appendChild (script);
    };



  [1]: http://math.stackexchange.com/
  [2]: https://gist.github.com/BrockA/5761948/raw/Add_Timeline_Button_to_Stack_Exchange_questions.user.js