The experimentation marches onward today, as YouTube has introduced the option for users to watch either a pre-roll ad (called a Promoted Video) or several in-stream ads. This will affect the longer, full-length content on the site (aka YouTube's ever-growing number of shows).
Essentially, before any ads or video start to play, the option now exists to choose to watch one longer commercial or to watch several smaller ads throughout your video. It's currently only offered on a small percentage of video plays, but we can expect this to increase if the tests are successful.
This type of testing is already prevalent on YouTube competitor Hulu. I consistently have chosen to watch Mario Lopez talk to people about how great Philadelphia Cream Cheese is for a full 90 seconds rather than get interrupted by intermittent ads. It's just my preference to get ads out of the way.