Hiya,
I am currently testing some DR scenarios.
As of 4.1 (thanks God for that), the datastores changing its status to 'inactive' when the LUN is gone, whether the reason is a failed SAN or simply the LUN is set to be offline.
Anyway, now I got 50 VMs running in a LUN which I set offline on SAN level. It didn't take long for the vcenter to notice that the LUN is inactive, but the guests are still 'appear' to be online. (i.e. io meter is still running etc.) but I am unable to do ANYTHING with it.
The most important thing I can't do : shutdown / power off ..
How long does it take for those VMs to actually disconnect from the virtual center ? I did try a rescan already but I wonder if there is any way apart from rebooting the host ... Also a clean reboot doesn't seem to be possible .. stucks ages on "Updating Summary Log" so I rather find a non-reboot solution 
As far as I remember vsphere checks every half an hour for diskspace for example, but its a lot longer than 30 minutes now and the VMs still appear to be running .. well ... 'running' ....