Mainstream support for SQL Server 2008 and SQL Server 2008 R2 is scheduled to end on July 8th, 2014, with Extended Support continuing to July 9, 2019. Microsoft is now promising a last Service Pack for these SQL Server versions. The Service Pack will basically be a rollup, packaging together Cumulative Updates and providing no new functionality.
The Service Packs will be aptly names SQL Server 2008 SP4 and SQL Server 2008 R2 SP3, however they will not be ready to ship until after mainstream support ends. If security issues are found after the Service Packs release, Microsoft will make those available, but you can expect no further cumulative updates made available for these SQL Server versions.
Hotfixes will still be developed, should the need arise, but only for those customers with Extended Hotfix Support Agreements in place.
There is no defined date as to when the Service Packs will ship. Microsoft is only suggesting that they will be made available sometime later this year.