Doesn't add "ui-sortable-disabled" to the class, but
$(".ui-selectable").selectable( "disable" )
Does. This is kind of inconsistent and confusing.
Having said that, I wish it wouldn't; I want to disable the control so that the user can interact with controls underneath (not sure if this is the best way yet), and the CSS style for ui-sortable-disabled sets some parameters that are inappropriate.