Is YouTube down?

For the past 2 hours, I've been unable to get on to YouTube.

It's on the scale of unlikely that  a service with the administration and infrastructure of google would go down, but it's only a year and a half since it happened last time (which was the first time).

So is this the second of biannualish youtube failures? 

Update (2:55) - The site is back up and running. Here are some interesting theories on what happened:

• Server maintenance to add AddSense
• DNS accidentaly remapped
• DNS hijacked