I don't think chord progressions can really be considered "rip offs" unless, as displayed by cracker-jack whitey Vanilla Meth up there, it sounds almost indecipherable from the original. But, throwing a stick in the spokes of this whole thing in terms of Hip Hop, sampled music is practically one of the foundations of it. Whether it's credited or not is another thing entirely.
Extending beyond that to artists simply using common chord progressions, I think I'd draw some sort of arbitrary line to say we can't really copyright a chordal sensibility without stepping on someone's toes at one point or another.
__________________
|