Per user (or session ID) per theme reordering; blocks can be disabled from drag and regions can be disabled as drop areas (ATM, only one set of "drop areas", ie can't say this can be dropped in a, not b, but this can be dropped in b, not a. This overrides the theme_blocks (not individual block themeing, but the themeing for all blocks in a region; unlikely that a theme would be using this is my guess).
