I have created a patch to allow the postion of a dialog to be specified using a position object.

With this patch you can do the following:

$("#dialog-modal").dialog({
 height: 140,
 modal: true,
 position: {my: 'center', at: 'center', of: '#test_form'}
 });

The patch does not effect the results of the Unit Tests for the Dialog, and all of the original options for the dialog's postion option should function as expected.

The patch is against the master branch, alternatively the a branch with the patch included can be obtained from: http://github.com/titan098/jquery-ui/tree/fix-dialog-position