This (dated) Slate article makes a pretty convincing case that when a proper fair use case finally gets to court, most sampling will likely be deemed fair use:

http://www.slate.com/articles/news_and_politics/jurisprudenc...

Artists like Girl Talk are getting bolder and more blatant in their uncleared sampling. It might have something to do with his giving away his recent albums, though.

Edit: Here's a couple other Slate articles that go into more detail -

http://www.slate.com/articles/business/moneybox/2012/05/adam...

http://www.slate.com/blogs/moneybox/2012/05/08/paul_boutique...