Adam,
As far as I know, these "holes" are in fact hair-thin (but deep) cracks caused by "stress corrosion cracking" in the weld material of PWR reactor vessel penetrations (nozzles) for the control rod drive mechanisms (CRDMs).
Of course the way the media report the story, you'd think its a foot-wide gaping hole ! (its possible the NRC used the word "hole" too, but they're used to seeing barely detectable microscopic fissures on x-ray film, so for them a crack several inches deep is a "hole").
This reactor vessel head nozzle cracking issue dates back to the late 1990s, and a number of US NPPs have had their vessel heads inspected since then ( for example Ginna and Oconee Unit 1), with much less severe indications than those at Davis-Besse. Analysis for those plants concluded that the results were within expected variation of models. For some reason, the situation at Davis-Besse seems to be quite a bit worse.
The inspections also revealed boric acid deposits on vessel head surfaces (boric acid is used for reactivity suppression -- enriched fuel in those reactors has lots of excess reactivity). Reactor vessel penetrations are difficult to inspect because the entire head is covered with insulation.
As for potential consequences of a leak through a crack like that, I don't really see how its different from a crack in a small pipe anywhere else in the primary heat transfer system -- an event easily dealt with using the ECC system. I believe that its certainly not nearly as severe as other design basis postulated events, such as a main line guillotine break.
It should be interesting to see more details as the situation develops....
Jaro