I've just upgraded one of my sites to 3.0 RC2 and it's working fine as expected (i.e. nothing's changed).
I've enabled multi-site in wp-config.php and disabled the plugins but when I went to 'Network' I got a warning about changing the siteUrl to the root of the domain first.
I can't do that on this installation i.e. it MUST be in a sub-directory called '/Blog'.
Is WordPress 3.0 (RC2) with multi-site/network still possible with this configuration? I'm a bit worried about enabling network if it's going to break the existing site ...