Orac -
On the subject of special weapon penetration: although today's known weapons (e.g. B61-11) probably can't penetrate deep enough to avoid the blast venting into the atmosphere, one suspects planners would propose a variation on the current 'double-whammy' approach used by Paveways against HAS and the like, in which the first weapon 'opens the door' for the second. Use of a deep penetration weapon like the GBU-28 (or the high-speed penetrating weapons now in development) would probably go some way towards achieving this.
From a broader perspective, this question of the use of low-yield weapons has been an active topic of discussion within the services over the past decade. This by itself is a fairly fundamental shift (at least in the public realm), and indicates a sense of greater freedom to at least consider the issue now that the days of MAD are - by and large - over.