The vSphere MOB can be reached by pointing your web browser to either an ESX(i) or vCenter host followed by the path "/mob". You will be prompted for credentials as you normally would prior to gaining access to the MOB.
Previous to vSphere 4.1, the MOB would display managed entities using their Managed Object IDs (MoRef ID) as a way to uniquely identify these objects. This made it very difficult for developers to identify the specific object you are viewing without having to dig further.
I think these two changes will make it easier for developers to navigate the vSphere MOB and assist with understanding the vSphere API and it's managed entities.