Bluehost wrote a script to automatically update and then do a health check on more than two million websites. They were able to make the changes with minimal customer service complaints, in effect drastically improving the health of the websites on their infrastructure.
They've now open sourced the tools they used to make it happen, so that other hosts can take advantage of it as well. Bluehost certainly isn't the first host to run autoupdates on their service, but they are probably the biggest by a wide margin.
I hope that other hosts will take note of what Bluehost is offering here and start to take the steps to update old versions of WordPress on their own servers.
You can view the code they use directly, as well as the implementation guide for more. This is a great move, and not an easy thing to implement considering the big ship that EIG is and the red tape they must've had to go through to open this up. Kudos to those that were involved.