This feature was earlier a port-in-progress in the OurDelta builds, but wasn't included in the work for 5.1 that Monty and I did for MariaDB.
It's in stock MySQL 5.6. It's a useful feature, and clients still sometimes need it. The pt-slave-delay tool does the job, but it's nicer to have it in the server - and heck, the code already exists.
If we can get this merged into (for instance) 10.1, that'd be great.