Apple has released a knowledgebase article outlining a problem with the ATI X1900XT video cards that were included as CTO options in some Mac Pro systems between August 2006 and January 2008. People using these cards have experienced distorted video output and other visual artifacts, and it appears the problem was in the manufacturing of the video cards instead of an issue that could be addressed by a software or firmware update. Apple is offering those affected by the problem a replacement graphics card, but only if the card matches a specific serial number range by having "V6Z" in the last part.
Before checking the hardware for its serial number, first be sure your system has the X1900XT video card installed by opening the System Profiler utility and selecting "Graphics/Displays" in the "Hardware" section (the card model should be listed in bold at the top). If the X1900XT is listed, then proceed to check the serial number by following the instructions in the knowledgebase article (reposted here):
Remove the graphics card (follow these instructions).
Look at the sticker on the back of the card.
Verify that the last four digits contain V6Z (the card must have a fully intact and present label).
Contact Apple for a refund if your card matches these criteria. The replacement program will be good until January 31, 2011, or until three years after the original purchase date, whichever comes first.