Okay, Im currently doing a degree in music production and last year we had the 'business' module of our degree. Sampling was one of the primary focus of a lecture, so here's my two cents on the legal side of sampling.
Basically when sampling first came about, everybody groped around for what kind of deals had to be made. Alot of sampled records around that time were released before anypony tried to clear the rights, which in turn led to artists and companies having the attitude of 'well, if they catch me then I'll make a deal'. Which they did. But do you know who's rights need to be bought out? Obviously the record company owning the record must be paid but the publisher of the composition also needs to be paid. Publishers will always insist on owning a percentage of the sample copyright, royalties and income. But this percentage depends on how significantly you have sampled. If you lift an entire melody for example they can take 50% or more of your income from the song.
The thing is, the catch me if you can idea stopped abruptly in 1991 when a rapper I cant remember the name of sampled a Gilbert O'Sullivan song and got caught. Not only did the rapper get slapped on the wrist but the case was forwarded to the US attorney office for criminal prosecution. Any intentional copyright infringement is a criminal offence if not dealt with the proper way.
Now, I don't have a problem with sampling at all. Hay, I use it myself now and again. But I feel it needs to be used in the correct way, aka not completely ripping off the original artist.