In any cases where I've found my work being used without permission, a simple email has cleared everything up. Most times people don't even realize that artwork on the internet can have a copyright. This blog post about Lane Hartwell (and from there, you can find links to several points of view) outlines the dilemma of people who want to share their art on the internet but not have people use it for free simply because it's easy to right-click.
I'm interested to know - if you are an artist, do you care if others use your work in a youtube video? Can people use it as long as they link back to you? If you're on the other side - an appreciator (is that a word?) of art - do you only use work where people have used a Creative Commons license? Or does that not really enter into it?
As for me, I'm pretty easy - if you ask permission, 9 times out of 10 I'll say to go ahead and use it. But that's only if you ask first.