In general, I don't understand why April Fools jokes on the internet have begun on March 31st - but apparently that's now standard. I also struggle with April Fools internet 'jokes' because they are mostly just made up stories (easy to do - though cleverness is clearly difficult) - but that's what makes YouTubes 'prank' so great: they actively made a product change and, in doing so, poked fun at themselves. YouTube RickRoll'd their users and their videos - linking all their featured videos to that classic 80s music video.
It's debatable who's career was resurrected more by web 2.0: Chuck Norris or Rick Astley.
If you don't know what a RickRoll is, here is a great example. Yesterday's #1 story on Digg was titled "Lala (Tiki Bar) Showers Video", raked in nearly 7,000 diggs and linked to this video:
Flick'Rolled:
Some guy RickRolling his class: