That works for positive numbers > 0. If numbers can also be negative or 0, you can use this formula:
WHEN MIN(ABS(Value)) = 0 THEN 0
ELSE EXP(SUM(LOG(ABS(NULLIF(Value,0))))) * ((SUM(CAST(SIGN(Value) AS int) - 1) % 4) + 1)
END AS Product
It looks daunting, but it's not that bad.
The WHEN checks for at least one zero in the range of numbers to multiply and returns zero when that is the case.
The ELSE has two parts. The first part is the formula Jeff gave, but on the absolute number of the value (ignore the NULLIF for now, I'll get to that later), so this produces the product of the absolute values. The second part counts the number of negative values in the range, and if it's odd it multiplies by -1 (otherwise by 1). The trick here is that SIGN returns -1 or 1, subtracting 1 makes that 0 or -2, and summing that results in (-2) times the number of negative values - the remainder after division by -4 is 0 when there's an even number of negatives, or -2 when it's odd. Add 1 to get a multiplier of either 1 or -1.
The NULLIF is required to prevent errors. SQL Server can only evaluate the WHEN expression after grouping, but by then it has lost the individual values, so it will calculate parts of the expressions before grouping. One of those parts is LOG(ABS(Value)) - which would return an error for values of 0. The NULLIF replaces them with NULL (and because of the WHEN, the resulting intermediates will never be used).
Hugo Kornelis, SQL Server MVP
Visit my SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis