Using SQL 2005, PDO_ODBC and datetime fields is a royal pain. MSDN documentation on CAST CONVERT shows that there is supposed to be an implicit convert between character types and datetime types. That's true... until you put it in a stored procedure and use variable declarations.
For instance this fails:
declare @date varchar;
SET @date = '20080101';
SELECT cast(@date AS datetime) AS poo
While this succeeds:
declare @date varchar(19);
SET @date = '20080101';
SELECT cast(@date AS datetime) AS poo
The PDO Driver appears to attempt an implicit conversion and so it fails whenever you try to insert data into datetime column types.
So to workaround this nuance in SQL, declare a character column type with explicit width. Then your implicit type conversion will work.