As with most things, "the devil's in the details"...if you as an independent artist had permission from a songwriter to post your version of his work BUT don't have permission from the writer's publisher of any of the forms the songs may have been publicly issued in before you did your version, you still may not own the rights to post or otherwise publicly offer your version. And publishers are a lot more likely to cruise such places as MySpace looking for "thieves" as they deem artists that do unowned material.
To bring this home to Mudcatters, I've recorded for my own use my versions of some of Jed Marum's songs. He knows that and has heard them; he's fine with me helping "spread" his songs about to friends etc. But I don't have any approval from Boston Road that published Jed's albums, so I don't consider that I have any ownership of the songs which would allow me to meet all the legal tests to put those songs on the internet except in a non-public fashion. Consequently my public songs on MySpace (and ezFolk and SoundClick as well) are either original works or are public domain/traditional songs that I've done my version of.