How to get last identity row Inserted when using instead of trigger
In your instead of trigger, you definitely can get the inserted value... but not until after you've performed the insert.

USE tempdb;
GO

CREATE TABLE dbo.SmellThis
(
  id INT IDENTITY(1,1),
  name VARCHAR(32)
);
GO

CREATE TRIGGER dbo.SmellThis_First
ON dbo.SmellThis
INSTEAD OF INSERT
AS
BEGIN
    SET NOCOUNT ON;

    DECLARE @ids TABLE(id INT);

    IF NOT EXISTS 
    (
      SELECT 1 FROM sys.objects AS o
        INNER JOIN inserted AS i
        ON o.name = i.name
    )
    INSERT dbo.SmellThis(name)  
      OUTPUT inserted.id INTO @ids
      SELECT name 
      FROM inserted;

    SELECT id FROM @ids;
END
GO

INSERT dbo.SmellThis(name) SELECT 'Remus';
GO


Results:

id
----
1


Now clean up:

DROP TABLE dbo.SmellThis;


As an aside, you should never, ever, ever be using @@IDENTITY or IDENT_CURRENT() anyway. And SCOPE_IDENTITY should be reserved for situations where you know only one row can ever be inserted. A common misconception with triggers is that they fire per row, like in other platforms, but in SQL Server they fire per operation - so a multi-row insert using VALUES(),(),() or INSERT...SELECT - which SCOPE_IDENTITY would you be setting to your variable?