Hello Will,
see this link
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a0080a07203.shtml
you need to add an ARP entry if you want to ping from the C2960 switches, in order to make the users connected to the C2960 to be able to ping the virtual IP address you need to define a static CAM for the multicast MAC address entry that uses the uplink port towards C3750 stack
The reason is that LAN switches do not learn these multicast MAC addresses as it happens for standard unicast MAC addresses
>>>However, since the incoming packets have a unicast destination IP address and multicast destination MAC the Cisco device ignores this entry and process-switches each cluster-bound packets. In order to avoid this process switching, insert a static mac-address-table entry as given below in order to switch cluster-bound packets in hardware.
mac-address-table static 0300.5e11.1111 vlan 200 interface
gi0/x
Also use a multicast MAC address that is out of the multicast MAC addresses used to map IPV4 multicast addresses in order to avoid issues with IGMP snooping. The one in the example is fine.
Hope to help
Giuseppe