During yesterday's Notes 8 Open Mic call, a few questions were asked by customers regarding interoperability / compatibility of Notes/Domino 8 with previous versions.
Check out the technote, "Supported Configurations in Notes/Domino 8.0" (#1264690) for information on this subject. It describes the supported configurations of Lotus Notes/Domino 8.0 when interoperating with servers, template, and/or clients from other releases such as 7.x and 6.5.x.