We had VMWare support look into the problem. They were clueless as well.
The way it seems, LUN presentation etc. are OK, but the ESXi hypervisors somehow seem to have lost the vmfs filesystem driver/Kernel module. The support guy couldn't tell me how this could happen...
This is happening on the latest ESXi 5.1 GA build with HP customizations, upgraded from 5.0. The admin on site is installing one machine from scratch, support was confident the data would show up then.