I've noticed that an increasing number of YouTube videos are restricted to a limited number of countries, probably because the company that uploaded them doesn't have global distribution rights or because it wants to use different marketing strategies in other countries. Even if YouTube says that "this video is not available in your country", you can actually see it using a very simple trick: replace http://www.youtube.com/watch?v=VIDEOID with http://www.youtube.com/v/VIDEOID (VIDEOID is the 11-characters video identifier).
Example of video blocked outside US (Madonna - 4 Minutes): http://www.youtube.com/watch?v=I9ciR9qR1dU
To see it, paste this in the address bar: http://www.youtube.com/v/I9ciR9qR1dU
Update (one day later): YouTube fixed the player and you can no longer bypass the country restrictions using this trick. Youtubegir has a nice proxy for YouTube videos.
The same trick works if you don't want to log in when you get this message: "This video or group may contain content that is inappropriate for some users, as flagged by YouTube's user community. To view this video or group, please verify you are 18 or older by logging in or signing up."
http://www.youtube.com/v/VIDEOID sends you to the player used by YouTube when you embed the video into a web page and this player doesn't perform country verifications and can't detect if you're logged in.
39 minutes ago