How do you disable account activation in WPMU and then log the user in right away?

In a WPMU setup the account information is sent to the wp_signup table before being passed to the wp_users table.

an easy fix for this is:

function your_disable_activation( $user, $user_email, $key, $meta="" ) {
    // Activate the user
    $user_id = wpmu_activate_signup( $key );

    wp_set_auth_cookie( $user_id, true, is_ssl() );

    wp_redirect( /*redirect to */ site_url() );
    exit;

}
add_filter( 'wpmu_signup_user_notification', 'your_disable_activation', 10, 4 );