After updating to v3.0.6 I received this error: "Fatal error: Call to undefined function wp_get_current_user()"
and that's discussed here: http://wordpress.org/support/topic/fatal-error-call-to-undefined-function-wp_get_current_user-4?replies=31
I had to use the require_once('pluggable.php') fix in order to get WP to load properly again.