What’s the easiest way to change the default landing page for BuddyPress groups?

I’m looking to have the default view for Groups be the Forum component instead of the Activity component. What’s the best/easiest way to alter the default landing component/page?

[Edit – My original answer will only work in the upcoming BP 1.6]

Versions of BuddyPress from 1.6 onwards

function bbg_change_group_default_extension( $default ) {
    return 'forum';
}
add_filter( 'bp_groups_default_extension', 'bbg_change_group_default_extension' );

Versions of BuddyPress prior to 1.6

For the moment, you’ll have to use something like the following, which is a modified version of bp_core_new_nav_default() (a function that is broken in the case of groups, because of various slug-related stuff):

function bbg_set_new_group_default_subnav() {
    global $bp;

    if ( bp_is_group() ) {
        // Set up your new default
        $new_screen_function = 'groups_screen_group_forum';
        $new_default_slug = 'forum';

        $parent_slug = bp_get_current_group_slug();

        if ( $function = $bp->bp_nav[$parent_slug]['screen_function'] ) {
            if ( !is_object( $function[0] ) )
                remove_action( 'bp_screens', $function, 3 );
                    else
                remove_action( 'bp_screens', array( &$function[0], $function[1] ), 3 );
        }

        $bp->bp_nav[$parent_slug]['screen_function'] = &$new_screen_function;

        if ( bp_is_groups_component() && !bp_current_action() ) {
            if ( !is_object( $new_screen_function[0] ) ) {
                add_action( 'bp_screens', $new_screen_function );

            } else {
                add_action( 'bp_screens', array( &$new_screen_function[0], $new_screen_function[1] ) );
            }

            $bp->current_action = $new_default_slug;
       }
    }
}
add_action( 'bp_setup_nav', 'bbg_set_new_group_default_subnav', 999 );

function bbg_set_new_group_default_action() {
    global $bp;

    if ( bp_is_group() && !bp_current_action() ) {
        $bp->current_action = 'forum';
    }
}
add_action( 'bp_setup_globals', 'bbg_set_new_group_default_action', 999 );