Here is a link to the transcript of an Australian Federal court case from a few years back. It details (if you can sift through all the legal jargon) the case of a bunch of guys that released an album without acknowledging samples.

IDIOTS!

They got completely shafted (and rightly so) and have been ordered to pay a total of USD $128,398.23. in damages and legal fees. Let this be a warning, but i assume none of us are quite as stupid as these twats.

Check out the list of tunes they sampled.

Great Find... this is hillarious... Definitely not the way to go about sampling your music... Like seriously how stupid do you to be to do something like that... Just makes you think huh?

There is also something called "Fair Use". Several countries have such law, including Canada and US. I beleive in US it measured in 15 seconds (not sure on this), and in Canada, it's rather vague and determined by the use. Basically you can use a small fragment without any acknowledgement.

This is how I understand it, but I am not a lawyer and could be wrong.