D84 said:well, what he says is true but saying that the Grey Album release is evidence for it may be missing the point perhaps.
What happens is that the artist signs a guarantee to the label, independent or otherwise, that all samples etc on the master are authorised and legit and if they're not the artist will take the blame/costs. The label then signs a similar guarantee to the distributor and so if there's any legal problems they get shuffled back to the artist.
it doesn't happen quite so harshly, labels usually have dedicated sample clearers who clear samples on behalf of the artists, it's just up to the artists to pass on the details of all samples used. There are levels of contestation built in too when owners can't be found etc...
alot of sample heavy artists nowdays often cut a deal with music libraries, who often welcome big people using their libraries as it's part of their livelyhood, and it looks good, dangermouse did this with the uk bmg library for crazy for instance.