This is just off the top of my head, but to move an item from position P to Q, you would do something like this:

    update items
    set pos = case
        when pos = P then Q
        when P < Q and pos between P and Q then pos - 1
        when Q < P and pos between Q and P then pos + 1
        else pos
    end

Plus a where clause to limit the shuffling to the actual list being reordered, since you would keep all users' lists in the same table.