If am right, you want to retrieve the data for the last 6 months from today ?, for instance if you run the query today (31 Oct 2011) the system should be able to retrive the data between 1st May 2011 to 31 Oct 2011
If am right and the datetime column in my example is HireDate
select *
from employees
where CAST(floor( CAST( HireDate AS FLOAT ) )AS DATETIME) between CAST(floor( CAST( DATEADD("M",-6,GETDATE()) AS FLOAT ) )AS DATETIME) +1
and CAST(floor( CAST( GETDATE() AS FLOAT ) )AS DATETIME)