Now sorted, thanks to the peeps over at tweetmeme.
Make sure you have the latest version installed.
Make sure you have <?php wp_footer(); ?> just before the </body> tag in Footer.php
And make sure that you don’t have a plugin running that moves javascript around. My problem was that I was running the wp_minify plugin. It was minifying the javascript which was moving the <?php wp_footer(); ?> tag to the top of my source code. When the tweetmeme button looked for it in the footer it wasn’t there.
Solution? I turned off minification of javascript. The button returned instantly. You may have a similar issue.
Regards
James.