Please add communications-only groups, leaving the current group system unchanged (for build permissions, etc). The communications-only groups would be much more lightweight, not requiring permissions queries per sim or region, thus reducing bandwidth use. This new type of group would allow only Notices and IMs, and would perhaps also allow related events to be set Notify per group.
Users would be able to subscribe to an absurdly large number of communications-only groups, because these groups wouldn't have the network demands of the current group system.
They could be called Announcement Groups, to clarify their purpose.
Many businesses and other groups use two different groups anyway: One for announcements and one for build/manage/admin permissions. So allow this split to be more fundamentally supported by SL itself, aiding those businesses by removing the headache of the 25 group limitation.
Note that this feature request started life as a comment on another feature request: http://jira.secondlife.com/browse/MISC-208#action_38430