The University of Wisconsin Madison has filed a suit against Sony and Toshiba, claiming that technology being used in the creation of the PS2's Emotion Engine infringes a patent which was filed by the university in 1986.
Little information about the lawsuit or the patent it deals with is available at the moment, although we do know that the University is seeking damages and a halt to the use of the technology in question.
The PlayStation 2 is based on a MIPS processing core with custom extensions added to it to improve the efficiency of the console. This unit is known as the "EE Core" - while Emotion Engine is a term used to describe this component and several others which are integrated onto a single chip and form the heart of the console's processing power.
Toshiba had a hand in designing this component for Sony, and was originally involved in manufacturing the chips as well. It's thought that the lawsuit centres on the manufacturing process involved in creating Emotion Engine devices, not any technology on board the chip itself, and as such both Sony and Toshiba are targetted by the suit.
Update #1: Fixed references to Toshiba's involvement in EE design; added further information on the lawsuit.