I'm also having the same issue. Just received my 2x4x2.8 Mac Pro yesterday and have been having intermittent freezes with wild graphical artifacts all over the screen - sometimes I get lines, sometimes just pixels, other times I'll get strange blotches of graphics and text. I've been on the phone w/AppleCare for the past hour and now I'm on hold for a product specialist. I have a *feeling* that they know about this issue and the product engineering group is desperately gathering data to determine the specifics of the problem and least expensive solution. Reason: The tier-1 Apple support guy said "they want to speak to you directly, rather than thru me".
Anyway - I noticed that no one has posted any logs from the system.log file. Here's what happens to me when it freezes up:
1/25/08 2:07:23 PM kernel ** Device in slot: SLOT-1 **
1/25/08 2:07:23 PM kernel ** GPU Debug Info Start **
1/25/08 2:07:23 PM kernel 0x01019588 000090a2 00009588 00000000
......<snip - laundry list>.......
1/25/08 2:07:32 PM kernel 508:0x00000000