$.fn.tweetify = function() {
 this.each(function() {
  $(this).html(
   $(this).html()
    .replace(/((ftp|http|https):\/\/(\w+:{0,1}\w*@)?(\S+)(:[0-9]+)?(\/|\/([\w#!:.?+=&%@!\-\/]))?)/gi,'<a href="$1">$1</a>')
    .replace(/(^|\s)#(\w+)/g,'$1<a href="http://search.twitter.com/search?q=%23$2">#$2</a>')
    .replace(/(^|\s)@(\w+)/g,'$1<a href="http://twitter.com/$2">@$2</a>')
  );
 });
 return $(this);
}

<p><a href="http://twitter.com/seanhood">@seanhood</a> have you seen this
<a href="http://icanhascheezburger.com/">http://icanhascheezburger.com/</a>
<a href="http://search.twitter.com/search?q=%23lol">#lol</a></p>