A neat thing about this feature was that it was an outcome of a recent "hackday" here at YouTube. Hackdays are similar in spirit to "20% time" -- the amount of time in a week engineers at Google/YouTube are encouraged free think and innovate on projects they're passionate about. These efforts allow us to focus on launching smaller features that aren't on our official roadmaps but that we hope are still worthwhile for our users. Anyway, this hackday came with donuts, which were delicious and way too tempting for me to resist. I also had a strong personal motivation for figuring this out: I'm in a hip-hop dance group, and the troupe wanted to share practice videos without going through the process of becoming Friends on YouTube.
Even without any official announcement (until this post, that is!), uptake and usage of this feature has been strong. Here you can see the growth of private-videos-per-day using friend-to-friend sharing versus the new URL-sharing:
Mike Lambert, Software Engineer, recently watched a friend's private video.