Here is an interesting article from the Google blog about continuous, microscopic usability testing.

Search experiments, large and small

We figure out what works by running experiments - tiny tests for a small number of users which help us determine whether that feature helps or hurts.

Experimentation is a very powerful tool, and we use it very widely to test potential changes to search. At any given time, we run anywhere from 50 to 200 experiments on Google sites all over the world.

Thanks to Alain for passing this one on.