How To Select Data For A Given Month Or All Records

I posted this question last night and thought I had an answer.  I have a date field.  I want to be able to filter records by the month of the date.  To do this, I pass the integer of the month.  But I also want to be able to return all the records if no month integer is passed.  So functionally,

select * from table where (MONTH([AD ENDS]) = @month)
will return all records where [Ad Ends] is in January if @month = 1 and all records if @month is empty.  The solution I got last night was to use

select * from table where (MONTH([AD ENDS]) = ISNULL(@month, MONTH([AD ENDS])))
If @month is null, all records are returned.  I was focused on another aspect of the page when this was posted.  It worked in the designer, so I thought I was set.  This morning I realized I don't know how to pass a null variable in a querystring.  Since a querystring is a string, it probably can't be done. 
Another suggestions was to change this programmatically.   But is there a way to do this in the dataset?  I'm using SQL Server 2005, a strongly typed dataset and the designer.
Diane 

 
The answer i got last night was to