VMware just changed the way to obtain an ESX Virtual Infrastructure Node (VIN) evaluation. An ESX VIN is composed by a single ESX Server, the VirtualCenter server and the VirtualCenter agent for ESX.
Till today obtaining an evaluation copy of both product was quite complex, interacting with VMware Sales folks or the worldwide distributors and waiting for a manual activation of your account.
Now this procedure is much simplified thanks to an online webform, automating the account activation for both products download.
Request your VirtualCenter and ESX Server evaluation copy here: http://www.vmware.com/programs/7/ESXVinEval.do
Update: the evaluation form finally provides access to ESX Server 2.5.2 and VirtualCenter 1.3.0