At the Oracle Open World 2011 in San Francisco Markus Michalewicz (Senior Principal Product Manager) announced in two sessions that there is a behavior change introduced with the 11.2.0.3 Patch Set regarding multicasting  requirement for the cluster interconnect network. In the version 11.2.0.2 lack of multicasting support for the cluster interconnect lead to a problem during installation, upgrade or generally Grid Intrastructure startup on the second node. With 11.2.0.3 lack of multicasting support will not lead to an error, rather as an alternative broadcast will be used  (only during cluster bootstrap to find the cluster peer nodes; after that the communication will be switched to unicast - as used to). Nevertheless multicasting is still recommended and with the 11.2.0.3 installer verified as part of the prerequisite checks (in contrast to 11.2.0.2):

INFO: *********************************************
INFO: Multicast check: This task checks that network interfaces in subnet are able to communicate over multicast IP address
INFO: Severity:CRITICAL
INFO: OverallStatus:SUCCESSFUL
INFO: -----------------------------------------------

BTW: Before upgrading the Grid Infrastructure from 11.2.0.2.X to 11.2.0.3 make sure you applied the patch 12539000 (ASM UPGRADE FAILED ON FIRST NODE WITH ORA-03113). This will be verified by the installer too:

INFO: *********************************************
INFO: Oracle patch:12539000: This test ensures that Oracle patch "12539000" has been applied in home "/u00/app/grid/11.2.0.2".
INFO: Severity:CRITICAL
INFO: OverallStatus:VERIFICATION_FAILED
INFO: -----------------------------------------------