I was writing my own re-tweet class and found that I need to crunch URLs on the fly to make them fit within the 140 character limit on Twitter. Come bit.ly and their API, easily accessible via REST and with a JSONP callback interface, just the job for extending Request.JSONP:

Request.bitly = new Class({
    // shorten urls via bit.ly
    Extends: Request.JSONP,
    options: {
        log: !true,
        bitly: {
            api: "R_e744c730bdde44a7eacc88cb892074e7", // GET YOUR OWN API KEY
            login: "webtogs2" // GET YOUR OWN LOGIN
        },
        url: "http://api.bit.ly/v3/shorten?login={login}&apiKey={api}&longUrl={longUrl}&format=json"
    },
    initialize: function(loc, options) {
        this.parent(options);
        this.options.bitly.longUrl = loc;
        this.options.url = this.options.url.substitute(this.options.bitly);
    },
    success: function(data, script) {
        this.parent(data, script);
    }
});

This is the class, now for the example uses, this will convert all links from the page into bit.ly URLs (if they are shorter than the original)

document.getElements("a").each(function(el) {
    new Request.bitly(el.get("href"), {
        onSuccess: function(data) {
            if (data && data.data && data.status_code == 200) {
                var orig = data.data.long_url, hash = data.data.url, difference = orig.length - hash.length;
                
                if (difference > 0) {
                    el.set({
                        href: hash,
                        title: hash + " - saved " + difference + " chars"
                    }).addClass("shortened");
                }
                else {
                    var error = "ERROR:\n your bit.ly hash '" + hash + "' is longer than the original '" + orig + "'";
                    el.set("html", el.get("html") + error);
                }
            }
        }
    }).send();

});

See it all in action on the little jsfilddle here: http://www.jsfiddle.net/dimitar/xVtZy/