i think you are incorrect as from this simple example script (im using sql2005)

declare @t table(i int, x int)

insert into @t

select 1, null

union

select 3, 1

union

select 2, 1

select *

from @t

select count_big(*)

from @t

select count_big(x)

from @t

it shows that count_big ignored the nulls in the second query