MICROSECOND being lost in stored function

[Larry D. Wilson]

I've been hammering on this all day, and I suspect either I'm doing something really dumb, or this is a bug - wanted to run it by all the gurus out there before I submit it.
When a timestamp or time value is passed to a stored function, it's not retaining the MICROSECOND value.
How to repeat - first create a test function:
DELIMITER $$
DROP FUNCTION IF EXISTS `f_test_time`
$$
CREATE FUNCTION `f_test_time`(
p_time TIME)
RETURNS INT
BEGIN
RETURN MICROSECOND(p_time);
END;
$$

And then run:
DELIMITER $$
SET @time = CAST('2010-10-14 14:21:27.019473' AS TIME);
$$
SELECT `f_test_time`(@time);
$$
SELECT MICROSECOND(@time);
$$

The first statement calling the function returns 0, yet the simple select statement returns the microseconds.
Anyone see anything weird?
And thanks for taking a look at this.