Okay, I upgraded four separate websites, each on a different domain, on the same server to WordPress 3.0 and all seems well.
I added to wp-config:
define(' WP_ALLOW_MULTISITE ', 1);
... but Tools is unchanged and looking around, I can't see anything relating to multi-site administration.
Is there anything I should be taking advantage of now so that I have a single administration instead of four seperate ones?