The case I remember most is George Harrison getting sued for "my sweet lord" the first three notes.."my sweet lord" are the same as.. "he's so fine". Otherwise I don't think the songs have anything in common, except those three notes repeated.
the writers or copyright holders of "he's so fine" sued and won a judgement agains Harrison.
I'm no lawyer but I'm gonna guess that there are all kinds of protections built into the law that protects the copyright holder against any trick, or variation you can think of.
to read more about it http://abbeyrd.best.vwh.net/mysweet.htm

This article also explains a little about how they decide how much to pay the writer and the different types of fees such as performance, mechanical, etc.

[This message has been edited by mikeathome1 (edited 01-08-2005).]