MPEG4 distribution question

I recently bought a Sansa View, because I like to be able to carry music and video around with me wherever I go. However, when I started to search for files that my Sansa could play, I found that there are very few sites that offer quality MPEG4s for download, and ever fewer that have content that I’m interested in. I finally broke down and converted my own videos.

My question is, is it illegal to distribute things like music videos, even if you are offering them in a format that the artist doesn’t distribute? I was thinking that maybe this was why I couldn’t find anything, or maybe it’s because nobody can be bothered to convert videos…Either way, I’d like some info. If the latter is the case, it may be a cool side project to run a free mobile downloads site…