Adriano, linked mode is a fuction of vCenter 4.0 to provide central management accross multiple datacenters where each datacenter has its own vCenter server. When you log into a linked mode vCenter you will have access to all vCenter servers across datacenters that are linked. Each linked mode vCenter is still a single maangement source for the datacenter it is managing and link mode does not provide any redundancy (ie if a vcenter fails, another linked mode vcenter will NOT take over).

Heartbeat provides the redundancy of vCenter by protecting the vcenter application and components including SQL databases. Heartbeat ensures that vCenter server will remain available for management, linked mode and invested VMware technologies. Heartbeat is sold as an addon to an existing vCenter license (or bundled with a new vCenter license), being based on a cloned active/passive model you will NOT need a second vCenter license or a second SQL license, just a second OS license for each instance of vCenter server being protected..

Justin King
Product Specialist - VMware vCenter Server Heartbeat

http://www.vmware.com/products/vcenter-server-heartbeat/