Since i can remember Creative Commons has struggled with coming up with a clear-cut definition of what non commercial use means in the framework of the Creative Commons Licenses. the current attempt to clarify this issue has resulted in a set of non-commercial guidelines, which aim to introduce more clarity. i have the suspicion that the current version is way to complex for most people to understand, which is supported by evidence here at WOS4 in berlin:

Contrary to what this picture from the conference venue lavatories suggests, a voluntary tip-yar does not constitute commercial use.