Noticed over on TeamRadeon
(an ATI fansite judging by the name) they got some ATi response regarding ATI's performance in Doom 3:
"Hi Richard - this is a non issue - Doom 3 isn't even available yet, & we all know that some of our competitors use partial precision where possible. We expect to have updated drivers available in the coming weeks."
. So, ummm, basically ATi will (think they'll) have better performing Drivers out shortly & NVIDIA's speed can be attributed (in part) to them using only partial precision.