you have to have vCenter 4.0 take over the vCenter 2.5 license server. Administration Menu > Settings... the first page has a spot to plug in the License server.
We switched licensing types between 2.5 and 4.0, so if you have ESX 3.5 or older in vCenter 4.0 ... you'll still need a license server somewhere to manage that component.
Regards,
Jon Hemming, B.Sc., RHCT, VMware vExpert 2009
http://twitter.com/vJonHemming
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