I've actually had to do this twice now. See here: http://communities.vmware.com/message/1524543
VMware support recommended that I do a storage rescan on all hosts after the data stores had been taken offline. Unfortunately, that resulted in more problems for me - when the LUNs came back online, the ESX hosts could see them but would not recognized them as valid data stores. So I had to re-signature my data stores - it's not difficult but it's nerve racking.
The 2nd time I did it, I didn't do the rescan - just left the hosts alone until my storage downtime was fully over, and then I did a rescan.
So my recommendation to you is that the safest thing to do is to power off your ESX hosts all the way down!
But if you don't want to do that, the next best thing is just power off the VMs, and don't touch your hosts after that. Only do a rescan after the LUNs have come back online.
You might have to do a re-signature anyway - it may be out of your control. Based on the fact that you say you don't want to add all your VMs to the inventory again, I'm assuming you are somewhat familiar with the resignature procedure.
