Latest incoming news on @BBCWorld confirms the Fukushima #2 reactor, the one whose core was completely uncovered this morning, has exploded. The explosion is reported by TEPCO officials as having been located "near the pressure vessel" [primary containment, the vessel itself] and to have damaged the suppresssion pool, a.k.a. the torus.
TEPCO spokesperson reported at a press conference that personnel are being evacuated from the site, they will leave 50 employees behind to tend the three destroyed reactors. Total meltdown for all three now admitted to be likely. The buildings are open to the environment, and there are still no reports about what has happened with the three reactors' inventories of spent fuel. The spent fuel pools were located in the upper portion of the secondary containment buildings (the big rectangular ones that now have no tops). They are not visible in overhead photos.
If the most recent explosion was of the reactor vessel the melting core is now exposed to release contaminants to the environment directly. Just wanted to post this breaking news, will keep following for any new reports.
Here's the BBC link.