Correct me if I'm wrong, but once music is published, does it not become part of teh public domain of information? I don't believe artists need to credited nor have their permission given for sampling or covers. What makes these songs rip offs? Is it the fact that a sample has been used? Or a cover even?
The masters are copyrighted. When you get sample clearance, you have to pay for access to DAT's of the masters. For example, Ghostface Killah - Holla samples The Delfonics - La La Means I Love You, and Kanye West - Through The Wire samples Chaka Khan - Through The Fire, and both artists got permission and paid for access to the masters.
It's much different with rock though, since with a little tuning of your guitar you can reproduce the meter and chord progression perfectly. The "melody" or "rhythm" is copyrighted, and it's up to a judge to determine if an artist stole it from another without permission, in which case they have to pay royalties along with some other fees. Garbage - Push It sampled the Beach Boys - Don't Worry Baby, and they just asked Brian Wilson for permission rather than trying to gain sample clearance through legal means, and he gave them the ok.
Then you can get into the world of interpolations, where it sounds like an artist ripped off another artist, but it's not noticeable enough to sue over. For example, Sublime - What I Got and The Beatles - Lady Madonna, or Eagles - Hotel California and Jethro Tull - We Used To Know. They sound similar, but it's nearly impossible to prove they knowingly stole the melody/rhythm/bassline/etc.
Does Vanilla Ice - Ice Ice Baby/Queen - Under Pressure count?
Yep. Queen sued and they settled out of court.