BUG #5711: input out of error with haversine formual

The following bug has been logged online:

Bug reference:      5711
Logged by:          Vince
Email address:      vincecarney@gmail.com
PostgreSQL version: 8.4
Operating system:   Linux
Description:        input out of error with haversine formual
Details:

The following will return an input out of error as the acos() function
cannot be -1 <= x <= 1.

SELECT * FROM
                (SELECT *, (3959 * acos(cos(radians(37.7438640)) *
cos(radians(37.7438640)) * cos(radians(-97.4631299) -
                radians(-97.4631299)) + sin(radians(37.7438640)) *
sin(radians(37.7438640))))
                AS distance
                FROM foo) AS distances
                WHERE distance < 10
                ORDER BY distance

If I break this down the following returns 1:
SELECT (cos(radians(37.7438640)) * cos(radians(37.7438640)) *
cos(radians(-97.4631299) - radians(-97.4631299)) + sin(radians(37.7438640))
* sin(radians(37.743864000)));

acos(1) would give me 0.

Thoughts?