I am using JQuery ui with datepicker and when a user tabs on to a field, they appropriately get the calendar pop up.
If you really don't care about the tab order, that is, not having it assigned so it follows the flow of the page, you could do something like this.
jQuery('.date').datepicker({
'onSelect': function(){
$(this).nextAll('input, button, textarea, a').filter(':first').focus();
}
});
It works on the assumption that when the user selects the date, he is finished with that field and moves on to the next one, so it simply selects the next field.