Here is a demo of a setup I did with ESX 4.1 and ESXi 5.0 in the same cluster with HA and DRS enabled managed using a vCenter 5.0.
Please note that upgrading VMware Tools to the latest version will still be supported on lower version of ESX/ESXi servers as it only update the OS drivers. However, upgrading the virtual hardware will only allow it to be supported on the latest ESXi server.
In such, that in consideration when doing your migration and upgrading of your vSphere environment and perform the virtual hardware upgrade last if possible unless you have enough resources to cater for your HA.
Update 19th Apr 2013
Refresh the video as first half has missing audio.