Pickett v. Prince
207 F.3d 402 (7th Cir. 2000) - full text opinion
Born Prince Rogers Nelson, Prince has changed his name several times over his career. On June 7th, 1993, Prince's 35th birthday, the artist announced that he would change his name to the now famous symbol. By the year 2000, he had changed his name back, but kept the glyph as his personal emblem.
The origins of the symbol are unclear, but it appears to be a modified version of the ancient alchemic symbol for the mineral soapstone.