While I can't find a link, I seem to recall Microsoft moving to a "no new features" policy for Service Packs at (enterprise) customer request. This seems sensible, as Service Packs aren't merely "security rollups", but also product lifecycle checkpoints:
When a new service pack is released, Microsoft will provide either 12 or 24 months of support for the previous service pack, varying according to the product family (for example, Windows, Office, Servers, or Developer tools). When support for a service pack ends, Microsoft will no longer provide new security updates, DST updates, hotfixes or other updates for that service pack.[1]
When a new service pack is released, Microsoft will provide either 12 or 24 months of support for the previous service pack, varying according to the product family (for example, Windows, Office, Servers, or Developer tools). When support for a service pack ends, Microsoft will no longer provide new security updates, DST updates, hotfixes or other updates for that service pack.[1]
[1] http://support.microsoft.com/lifecycle/?LN=en-us