I can think of two approaches:

SELECT *
FROM table
WHERE DATEPART(yy, datetimecolumn) = DATEPART(yy, datetimeconstant)
AND DATEPART(dy, datetimecolumn) = DATEPART(dy, datetimeconstant)

The downside with this solution is that it's not optimizable.

The other solution is to have the front-end supply the constraint in a bounded form:

SELECT *
FROM table
WHERE datetimecolumn BETWEEN '19990811 00:00:00.000' AND '19990811 23:59:59.999'

Which is optimizable but requires the front end to format the constraint propertly.

-Mike