The good news is that Vista SP1 is now publicly available, and that installing it (after uninstalling the SP1 beta) has removed the "Evaluation version" displayed on my desktop. I realise there were other ways to remove it, but I wasn't really interested. Why? Because fuck you, that's why.
The bad news is that the process of uninstalling the beta and installing the full version took almost an hour and a half. That's a lot of painful downtime for an update which I really only applied out of perverse interest..
* Disclaimer - I only run Vista on my work desktop. I'd never use it at home.