I get a lot of PMs asking how to blur/mask faces. I have Adobe Premiere 2 and Sony Vegas 7. Both of these programs do the same. It's not easy, and they are not cheap. But if you really have alot to put out and want to blur, these will do the job. Also, I just got my hand on Magix Movie Edit Pro 14. I am almost sure it does it, but I'm using the free trial, which lasts up to 60 days. I like this program, but using the trial version makes it hard to find documentation on how to use. I'd like to know what other people are using. Let's talk about it.

I have moved this post from the "Video" section to the "Fliming Technique" section, where it is more appropriate.

As far as I know, that could be done with Adobe After Effects automatically (i.e tracking head), but i guess that would be a little too much for many users.

You can find some info there:
http://www.mediacollege.com/video/special-effects/blur/

No matter what program you use, most of the time you copy the video track, blur the copy with whatever transform effect you want and them crop it so only the part you want to hide is blurred.

Not always easy if the part you want to blur is moving but with keyframes, it's possible. It's just take time if there is a lot of non-linear movements.