In reply to Jamie B:
Very true, and there's an argument to be made for that. But in proper winter conditions (rather than silly Alpine ones where, admittedly, the cornice persists) the cairn is sufficiently iced/snowed up to allow easy retrieval of an ab rope, particularly as no knot will be required since a single 50m rope easily gets you over the cornice.
Re snow bollards, an argument against installing bolted cairns is that it will keep this incredibly useful and somewhat lost art alive. Nowt more bomber than a well dug bollard!