Optional parameters and date index

I'm playing around with two very simple queries. There is a non-clustered index with StartDate and EndDate, as well as Id as an included column.

DECLARE @startDate DATETIME, @endDate DATETIME
SELECT @startDate = '4/1/2011', @endDate = '5/1/2011'

-- Does Index Scan (slow)
SELECT Id
FROM dbo.Table
WHERE
    (@startDate IS NULL OR StartDate >= @startDate) AND
    (@endDate IS NULL OR EndDate < @endDate)

-- Does Index Seek (fast)
SELECT Id
FROM dbo.Table
WHERE
    (StartDate >= @startDate) AND
    (EndDate < @endDate)

Is there any way to rearrange, pre-calculate, or otherwise change the query to have an index seek occur in the first example?

Edit: I know this is a very basic indexing problem, but I haven't found a good solution yet. Note that I am declaring the variables, b开发者_如何学Pythonut those would be parameters in an sproc.

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What about the following?

DECLARE @startDate DATETIME, @endDate DATETIME
SELECT @startDate = '4/1/2011', @endDate = '5/1/2011'

SELECT Id
FROM dbo.Table
WHERE
    StartDate >= ISNULL(@startDate, '1/1/1753') 
    AND
    EndDate < ISNULL(@endDate, '12/31/9999')

This code is probably broken if you have an end date of 12/31/9999 in your table that you actually want returned from your result set, but how often does that happen?