Straight from apple.com's MacBook Pro page on the Graphics
Pure digital signal
When you connect an Apple Cinema Display to MacBook Pro, you lose nothing in translation. That’s because the DVI connection gives you a pure digital signal from system to display. View more than 4 million pixels on the 30-inch Apple Cinema HD display, powered by the ATI Mobility Radeon X1600 and the dual-link DVI built into MacBook Pro. With Mac OS X, you get three options for using your display: dual-display mode, video mirroring, and lid-closed mode.