[jQuery] $.post error on response

I'm using jquery 1.3.2 and trying to send some data using $.post to a
PHP script. For some reason the response is not being returned and
Firebug is reporting an error on line 3633 of jquery.

function check_out_image(image_id) {
$.post('ajax/checkout_image', {imageid: image_id} );
}

A very simply function that POST's an image ID to my PHP script. The
PHP script is getting run and updating a MySQL database so I know it's
calling the right function and working but the echo response never
makes it back.

For reference line 3633 of jquery 1.3.2 is the xhr.send(s.data) line
below:

try {
xhr.send(s.data);
} catch(e) {
jQuery.handleError(s, xhr, null, e);
}

For testing purposes I changed my PHP function to a simple echo "foo"
statement with nothing else in it and it still doesn't get returned.
Any ideas on why the response wouldn't get sent back?

Thanks,
Dan