Here you go
DECLARE @Orders TABLE (OrderedDate DATETIME, Amount FLOAT, Currency VARCHAR(50))
INSERT INTO @Orders VALUES ('2012-02-07', 2.6, 'USD')
INSERT INTO @Orders VALUES ('2012-02-07', 1.3, 'GBP' )
INSERT INTO @Orders VALUES ('2012-02-09', 10.08 , 'USD')
INSERT INTO @Orders VALUES ('2012-02-09', 15, 'GBP')
SELECT OrderedDate, SUM(Amount), 'GBP' Unit
FROM
(SELECT OrderedDate, CASE WHEN Currency = 'USD' THEN Amount * 0.629846 ELSE Amount END Amount
FROM @Orders) As New
GROUP BY OrderedDate
I have one question though, did you try out something or posting it directly in forums so that someone else will do the work for you?