Late yesterday VMware made vSphere 5.1 generally available. Go to
to download. The free version of ESXi 5.1 is also already available:
And HP hardware owners will be happy to find that there is also an HP customized version of the ESXi 5.1 ISO available for download. The shortcut http://www.vmware.com/go/downloads-image-hp-esxi still points to the 5.0 version, but HP's ESXi 5.1 GA is available here:
Right now it is not yet possible to update an existing ESXi 5.0 installation to 5.1 using the well known methods (via Update Manager and the VMware Online Depot). You will probably be able to update by booting the host with the ESXi 5.1 ISO, but the other methods should also be available shortly.