They are using Dual-link DVI.
Go to the web page Apple.com
They are using a special NVIDIA card:
" The groundbreaking new NVIDIA GeForce 6800 Ultra delivers the industry's first 16-pipe superscalar architecture and support for the world's fastest DDR3 memory to raise the bar for 3D graphics performance. The specifications of the GeForce 6800 Ultra GPU, are stunning: Using over 220 million transistors it supports a 256-bit interface for an effective memory bandwidth of 35.2 GB per second which delivers 600 million vertices, 6.4 billion textured pixels per second. The GPU is built on an AGP 8X board and includes 256MB of DDR3 memory for use in the most demanding graphics applications. It is the first card available to support the DVI standard dual link digital signal specification from the two DVI ports it features . This capability is required to drive the new 30-inch LCD, high resolution Apple Cinema HD display. The combination of a GeForce 6800 Ultra with a dual processor Power Mac G5 driving two 30-inch Apple Cinema HD Displays is the definitive tool for the creative professional. "
It uses the DVI standard.