Notes from Facebook's Developer Infrastructure at Scale F8 Talk
The most interesting part for me was<p><pre><code>    > They appear to use code coverage to determine what tests  
    > to run. "We're not going to run a test unless your diff 
    > might actually have broken it."
</code></pre>
This seems like such an obvious optimization in hindsight, I'm surprised it's not more common. Does anybody know of other CIs that can do this?