I cannot believe no one has posted this yet, but just to clarify the weapon's characteristics:

12,000 ft/sec = 3,657.6 m/sec (or about 3.7 km/sec) - according to the original post. However, a bit of googling revealed that the rods could be travelling at up to 36,000 ft/sec at impact.

36,000 ft/sec =10,972.8 m/sec (or about 11 km/sec).

So, the apparent upper bound is 11 km/sec, apparent lower bound is 3.7 km/sec.

The suggested dimensions are 20 ft long by 1 ft diameter. On this detail most accounts are similar, as is the proposed material - tungsten. As has been pointed out by formulaterp, this amounts to a gross vehicle weight of about 8,500 kg. However there seems to be disagreement about how much of this mass will survive reentry. All things considered, I suspect that re-entry losses would be minimal. Nevertheless, let us presume 100% survival to target as the theorectical maximum, and 50% survival to target as a minimum.

So, we have an object of between 4,250 kg and 8,500 kg travelling at between 3.7 and 11 km/sec.

Unless I've miscalculated (and it's been known to happen) this gives us a worst case yield of 29 GigaJoules and a best case yield of 514 Gigajoules. Stated in terms of the ever popular "tons of TNT" this is between 6.3 tons and 123 tons.

On the whole, given the purported accuracy, I would see this as a pretty good bunker buster, but also as a very effective ship killer.

Sheki