If you are looking for

• the SAN attached hosts:  check on FC switch side, the active zoneset on each fabric. If a good naming convention is used it can help. Check also with the cable guy, he surely has some documents of which host is connected on which FC switch.
• the 'masked' hosts on symmetrix (i.e. which host access which volume) check the symmask or symaccess db.

yes

each host will have a last modifed or last updated field against its master agent

so just click on cog in bottom right of console and sort my last modified or whatever, this should show any dead hosts

Can you clarify if this was an upgrade or a complete new 6.1 install? And also if it was a new install are the new servers (most importantly the one with the ECC Server running on it) installed with the same IP address and host name as the old servers?

Its a new server with new ip and name.

Failing all other options, it is going to depend on what kind of documentation they keep on What is allocated to what host.

• If there is a good zone naming convention in place you should be able to generate a list of target hosts from the zoning database
  
• If the arrays are CLARiiON and the naming convention used in the initiator registration (at least for manually registered hosts) is good, then you should be able to pull a report of attached hosts off NaviSphere Managers
  
• If the array is Symmetrix and someone actually set up initiator aliases (may be a long shot) then you may be able to pull a list from SMC or Solutions Enabler
  
• If documentation is kept of allocations or switch port assignments you should be able to get your list from there
  

If all else fails then someone is going to have to start trolling the data center server racks for hosts with fiber running into them