Right now I have two separate vCenter servers (one older, one brand new), each managing 3 clusters of hosts. I will eventually need to get 300-500 VM's from the old clusters to the new. If I put the two vCenter servers in linked mode can I migrate VM's across clusters on separate instances of vCenter?
Edit - Can't believe I didn't think to mention the versions. The old stuff is all 5.1 and the new systems are 5.5.
You don't say which version you're running - if you're running version 5.x then you can't 'hot-migrate' but you can 'cold-migrated' (kind of, by either exporting/importing the VMs or sharing the datastores between vCenters and removing/adding the VMs), if you're running version 6.x and your vCenter's share a PSC then you can just go ahead and do it!