fwiw, I just used the following doc to install MEM 1.1.2 using Update Manager to several ESXi 5.1 hosts that did not previously have MEM installed, but did already have an iSCSI vSwitch configured and connected to an EqualLogic array. http://hansdeleenheer.blogspot.com/2011/07/equallogic-multipath-driver-for-vsphere.html#!/2011/07/equallogic-multipath-driver-for-vsphere.html
It installed and changed the default path selection from Round Robin to Dell_PSP_EQL_Routed but did not appear to alter any of my ESXi vSwitch config, which is good. Everything seems to be fine.
I'll update with more info after I perform a similar procedure during an upgrade from ESXi 4.1 to 5.1. The ESXi 4.1 servers already have an older version of MEM installed so this should be interesting. 