Tag Clouds, Zoom Clouds, MO Clouds
Jonathan very kindly mailed me and pointed me at ZoomClouds which would have done exactly what I wanted. Rather than using the Technorati API they use a Yahoo API (the content analysis API) and their own content analysis tool to automatically generate tags from your blog. Jonathan's written a comprehensive write-up of the key aspects of tag clouds.
Had I known about ZoomClouds earlier I would probably have used it - they use a similar technique of adding a script tag in your page that embeds a piece of JavaScript to render your own personal tag cloud. You can define certain aspects of your tag cloud on their site and they even have a simple API if you want to take things further.
Also, the description of the "font distribution algorithm" linked from Jonathan's blog above makes mine (MO Clouds) look very inadequate indeed. I will probably need to return to that at a later dates as I have visions of 800pt text with one character filling the tag cloud...
tags: tag cloud, zoom clouds