A couple variations (these will get you the same query plan as what rxm posted).

SELECT isnull(cast(id as varchar(12)),'Total'), sum(cnt)

FROM @rollup

GROUP BY id with rollup;

or...

-- since we're only grouping on one column

SELECT isnull(cast(id as varchar(12)),'Total'), sum(cnt)

FROM @rollup

GROUP BY id with cube;