It is true that WP and WPMU were merged in 3.0, but the MU, now called Multi-site, functionality has to be explicitly turned on. Since you don’t seem to have the wpmu_validate_blog_signup() function available to you (as revealed by the fatal error), I’m guessing you didn’t explicity turn on multi-site, or you didn’t do it properly. The WP developers added a few hoops to jump through to enable multi-site to discourage the casual user from borking their site. These hurtles will be removed in a future release, likely 3.1, when the developers have more time make it user-friendly for most users.
