Just found in the SQL Release Services Blog:

Update on the Service Pack plans for SQL Server 2008 and SQL Server 2008 R2

Dear Customers,
We are planning to ship one last Service Pack for both SQL Server 2008 and SQL Server 2008 R2.
Because of the maturity of SQL Server 2008 and 2008 R2, these Service Pack(s) will be an exception in terms of timing and will ship after mainstream support of these releases ends on July 8th 2014.
In scope, these Service Packs will be a packaging and release of Cumulative Updates and will contain no previously unreleased functionality. For further details on these Service Packs please refer to the FAQ below.

Regards.
Jean-Yves Devant Senior Program Manager

You will find the whole blog post (including the FAQ) in the SQL Release Services Blog.

Thanks Microsoft, this is the answer we wanted to hear!