With the new release of vSphere 4.1 there are some changes and news:
the terminology as changed and now ESXi is ESXi Hypervisor Architecture
the "old" ESX (or full ESX or legacy ESX or ESX with the service console) is available in the last release, from next version ONLY ESXi will be available.
VMware suggest to migrate to ESXi, the preferred hypervisor architecture from VMware. ESXi is recommended for all deployments of the new vSphere 4.1 release. For planning purposes, it is important to know that vSphere 4.1 is the last vSphere release to support both ESX and ESXi hypervisor architectures; all future releases of vSphere will only support the ESXi architecture.